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Aliens are invading with lasers, spacecraft, and tons of superior technology. The end of the world seems to be coming, until some kid finds the aliens one weakness. Nerf guns*.

For some reason the aliens seem to die when hit by a Nerf dart*. Shortly after discovering this weakness, the remaining military grabbed every Nerf gun they could find from Walmarts and other stores and starts pushing back.

But this raised a question, why can the aliens die from Nerf darts easily, but assault rifles need to shoot them at least a dozen times?

Alien details; Aliens are humanoid, equipped with laser(for most handheld) and plasma(for larger handheld and mounted) weaponry. They wear airtight suits because they can't breathe earth air and have energy shields that protects them from human handheld weaponry(except Nerf darts for some reason).

Humans; Humans are humans. Modern tech and standard post-apocalyptic status with everyone carrying a gun or dead. Not much to say there.


*- includes off-brand Nerf guns and darts

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    $\begingroup$ If you look at just about every alien-invasion movie ever: Because Aliens with high technology are stupid. They'll have extreme water allergies and invade Earth without protective gear on. They'll use hive-minds that can be easily identified and taken down in one concerted effort before all others die out. They'll forget that diseases exist and subsequently die from those on Earth (and somehow not kill Earthlings the same way). Let's introduce stupidity: Aliens use a Technomagical shield that is pretty good except it damages itself and the wearer if chemicals in nerfguns touch it. $\endgroup$ – Demigan Dec 23 '20 at 14:20
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    $\begingroup$ @Tortliena, knives are still blocked by the shield, but if the shield is down they can stab through the airtight suit if they have enough force. $\endgroup$ – Ceramicmrno0b Dec 23 '20 at 14:21
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    $\begingroup$ for some reason my ears are ringing with some horrid warbling singing, and the phrase "mars attacks!" comes to mind. $\endgroup$ – PcMan Dec 24 '20 at 6:38
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    $\begingroup$ This exact concept was a storyline in "Sluggy Freelance" back in 2001: archives.sluggy.com/book.php?chapter=26#2001-12-21 $\endgroup$ – Chris Sunami supports Monica Dec 24 '20 at 17:37
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    $\begingroup$ It seems I'm not the only one to immediately answer "An extreme and rapid allergic reaction to foam"... Nice. $\endgroup$ – Kaithar Dec 24 '20 at 19:48

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Alien shields are made of special compound which protects them very nicely against impact weapons like bullets by dissipating impact force to larger area and resisting punctures.

However, the chemical composition of Nerf tips is (as luck would have it) something that aliens have never met before (so it escaped their testing) -- in contact with shield compound, it starts chemical process very slowly at the point of impact, but then expands in circle faster and faster in runaway reaction consuming more and more of the shield.

The chemical reaction is not only exothermic (so it basically cooks the alien inside the shield), but it also causes the shield compound to enter phase change, in which shield compound expands into crystalline structure requiring bigger volume - with net effect of squashing the alien inside.

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    $\begingroup$ "Nerf foam is made from a solid, spongy cellular material produced by the reaction of polyester with a disocyanate, while carbon dioxide is liberated by the reaction of a carboxyl with the isocyanate." Reacting polyester with a disocyanate makes polyurethane, which I can't find anything that it will react with. Something in the alien's chemistry reverses the reaction and splits the CO2, freeing oxygen and starting a fire. (?). Anyway, chemical reaction, +1, because (if I was writing it) they literally eat lead for breakfast through osmosis, and have no skeleton like The Blob. $\endgroup$ – Mazura Dec 24 '20 at 2:22
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For aliens, or creatures with interstellar-level technology, boots-on-the-ground planetary invasion and subjugation of the local population makes almost no tactical or strategic sense from a military perspective. That's why their invasion isn't a military operation, it's an alien entertainment show:

The aliens have built remote-controlled bodies (or are just religious zealots who all dream to 'die' on live "TV") and are all live-streaming their invasion back to their civilization for views and that sweet, sweet, advertiser revenue. What they want to do is produce high-drama action and eventually, when the invasion is over (win or "loss"), bring the humans into the fold.

Unfortunately, many of the earlier civilizations that the entertainment-loving aliens "invaded" decided that against overwhelming alien firepower, escalation was the only option and they ended up deploying nuclear weapons or other WMDs. This, of course, left the planets they were invading irradiated or otherwise killed a large portion of the native population and "nuclear apocalypse" seasons quickly grew stale (irradiated wastelands all look the same to the viewers at home).

Because of this, the aliens instituted a new policy: namely being vulnerable to toy weapons rather than real weapons. Their reasoning is that this discourages escalation in weapon selection and makes it less likely that the natives deploy WMDs. At the same time, toy weapons don't significantly decrease the potential for heroics, daring charges, or clever tricks that the natives can try, and that all makes for good TV. Usually, of course, the aliens win, but underdog stories are also extremely popular and the aliens at home love to gamble about battles or even follow the actions of specific natives ("heroes" like generals or similar).

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    $\begingroup$ How funny your answer is complementary to mine. You could mix the two and replace the "doing war" stuff with "doing the showtime" one in mine, and explain how they achieve that with my answer in yours ^^. $\endgroup$ – Tortliena Dec 23 '20 at 18:19
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    $\begingroup$ I would love to watch this show! Even the answer itself is entertaining. Great answer! $\endgroup$ – KhanElmork Dec 24 '20 at 6:32
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    $\begingroup$ Makes (some) sense. The alien overlords may send in some of the conquered races to be used as cannon fodder. The suit themselves may be rigged to kill the wearer in case of a nerf gun hit. Morover: the OP states they are 'humanoid'. Maybe they were humans enslaved and bred millenia ago. Creepy. My issue is: why just nerf gun projectiles? what if one were to shoot the alien with a champagne cork (a la Julian Bashir, secret agent)? $\endgroup$ – Duncan Drake Dec 24 '20 at 9:47
  • $\begingroup$ Always amazing how people in this stack come up with incredible interesting answers to questions where I thought there is no good answer $\endgroup$ – Hakaishin Dec 24 '20 at 12:41
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    $\begingroup$ I don't imagine that this would stop people from deploying nukes if they're losing the war. If alien invasion movies have taught me anything, it's that nukes are the government's eventual solution to basically every problem, regardless of how effective or ineffective other weapons are. $\endgroup$ – NotThatGuy Dec 25 '20 at 11:01
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Non-metallic non-organic low-velocity projectiles.

It's a pretty well known fact that low velocity projectiles will pierce even the most developed personal shielding devices. See Dune shield tech or even the Go'uld shield tech.

Normally this means that a thrown hatchet, knife or other such low tech weapon will do the trick. However, your alien shield tech seems to stop all metals from piercing the shield devices as well. To be safe, the aliens also added natural organic products to the no-fly list, as most alien worlds they encoutered had some form of hard organic substances that could be used as weapons. So no throwing wooden/bone/shell arrowheads/stakes/javelins at them.

This is where the plastic nerf darts come in. The plastic/foam/non-metallic/non-organic projectiles pierce the shield tech with no problems.

So your nerf guns, designed to shoot these plastic projectiles at the low velocity required, are the trick to get through the highly sophisticated shielding tech.

The aliens, having not encountered this particular chemical arrangement have a simple allergic reaction on contact. First hit starts the allergic reaction which softens up the alien to a second low velocity round which goes through the alien body like a knife through soft butter.

Edit. Their airtight suits could have some sort of biochemical reaction to the plastic contact, rendering the suit faulty. This would allow toxic earth-air inside the alien's life-support system.

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    $\begingroup$ I think the suit would be the one to be "allergic" then, since they wear one ^^. $\endgroup$ – Tortliena Dec 23 '20 at 17:03
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    $\begingroup$ Yeah. Just realised I forgot to take suit into account. Added a quick edit. $\endgroup$ – EveryBitHelps Dec 23 '20 at 17:06
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    $\begingroup$ Errm. Your aliens might want to avoid any contact with our ocean water. What with all the micro-plastics in it, their suits might get severly corroded/damaged! $\endgroup$ – EveryBitHelps Dec 23 '20 at 17:15
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    $\begingroup$ Bonus plot hook: since the aliens aren't stupid, they'll gladly perform spectral analysis on the arrows, and the humans now need to rush before the spectral analysis is complete and the alien shield software gets updated to block plastic foam as well. $\endgroup$ – John Dvorak Dec 23 '20 at 18:15
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    $\begingroup$ Love this answer, but here is a slightly different take that allows plot expansion - Perhaps no one ever tried a bow and arrow? $\endgroup$ – Paul Smith Dec 23 '20 at 22:44
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NERF is really an acronym. Stands for Non Earthling Repulsion Force.

Nerf was invented by time travelers who knew about the upcoming invasion, and knew about the fatal flaw in their shields. Nerf makes q tiny crack in their air shield, and then the atmosphere kills them.

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Gosh, isn't it hard. Here's my try :

Alien shields have a terrible flaw

They are proportionally inversely effective against incoming impacts. Indeed, the shield's wavelength is accurately designed to deter any offensive maneuvers aggressively, not unsuspected friendly ones.

This means that high-velocity bullets are slowed to a stop, while slow, very light projectiles get insanely accelerated, transforming it into high velocity shrapnels and causing various traumas and cuts.

They calibrated their shield only against solid material, not liquids, and the snow that may reach it melt before by the intense heat it makes, so rain passes through without issues. Their suit is only corrosive and heat resistant, since yes, why bother wearing an heavy armor when you have an "invincible" shield, after all? And when they meet at the intergalatic coffee machine, they just switch off their shield, something no man has ever seen.

On their side, the soldiers didn't notice it because no one escaped alive to tell the tale, and even if they did, the commanders laughed at them, stuck in their thinking of "stronger is better". But that's exactly the strategy the aliens have seen from thousands of galactic warfare, so they prepared all their tools against it, forgetting this "minor" inconvenience.

EDIT : Some people wisely noticed in the comments a flaw in my shield reasoning concerning other light, slow objects. The best solution I personally have to circumvent this is to also consider that the molecular holes in the structure of the foam are the same length as the shield waves and so they enter in some kind of resonance, sending darts at a much higher intensity than other items. If you're still not really convinced (I can understand :) ), you should take a look at the other great explosive shield answers lying around to supplement mine!

Also, the Aliens targeting systems are overly sensitive to intense colors

When they see these flashy, orange-coloured weapon, the scanners become suddenly overly saturated and they need to change the sensitivity to pick-up new targets.

This explains why the kids have managed to kill the alien : It needed to have a lock-on to shoot. And this also explains why the UFOs stop midway with their weapons of mass destruction in vast cities : It's not because the weapon need to charge or because they feel confident, no! It's just because they need to recalibrate their scanner for a few seconds due to the too many and too much colored shirts and cars running around.

And while it could have been obvious, the military didn't notice it because they are used to wearing camo suits, which are totally useless against such "perfectly" calibrated scanners! It took one kid in a once cheerful toy mall to finally discover the truth about their nature : They only prepare for the expected. And you never expect your enemies to nerf their guns to win!

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    $\begingroup$ So a blowing leaf, or falling twig in the forest is also lethal to these shields? Or a bird or insect that tries to fly close will instead get turned into flechettes? Or a piece of paper / trash blowing in the wind in an urban environment? Or if they ever toss a tool to each other. All small and light, and non-rare enough that this problem would be noticeable. $\endgroup$ – Peter Cordes Dec 24 '20 at 5:38
  • $\begingroup$ @PeterCordes Ack, guess I am back to the alien black board... Maybe... No. This ain't working. Uh, then... Nein. This, niet. Ah, maybe two solutions? The dart's speed is just higher that most items carried by wind objects. It would be more a frequency set-up than the inverse shield. Or.... This only works with materials that have holes on a molecular level like foam... hmmm... I am not terribly happy with these explanations... $\endgroup$ – Tortliena Dec 24 '20 at 8:47
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    $\begingroup$ This gives new meaning to Butterfly Effect... as the breeze created by the flapping wings of a butterfly, although small... is devastating. "We have engaged the aliens and their crack troops. Release the butterflys!" $\endgroup$ – WernerCD Dec 24 '20 at 13:25
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Supply chain disruption and bad coding

The Setup

The enemy shields are completely impregnable, but use up a lot of energy. They go through one battery every ten minutes. The batteries, which are based on zero-point energy, use an exotic tech that causes them to explode violently if they get within six feet of each other, so soldiers can't carry multiples.

In order to keep them refueled, the mothership keeps a constant stream of tiny battery drones, roughly the size of a nerf dart, flitting back and forth between the mothership and each soldier.

The shields act as a notification layer between the drones. When an incoming drone is detected, the layer notifies the current drone to head out, then waits for the current drone to acknowledge the signal. Once it does, and begins to head out, the shields open to receive the new drone.

The AI responsible for per-planet calibration of systems did its usual comprehensive analysis of all military technology, flora, and fauna, and gave parameters for drone identification algorithms.

The algorithm, which was optimized by a black-box neural network, boiled down to (roughly):

condition_list = 
   [Shape.isCylinder, 
   length == (4.50×10^33 ± 5×10^31),
   diameter == (8.00×10^32 ± 5×10^31),
   mass == (45000 ± 1000),
   speed <= 2.5×10^-41]

(Units are naturally in universal planck dimensions)

There were a couple other conditions, but they ended up getting mostly optimized out or weighted more lightly by the AI, which after some testing found that this set of rules guaranteed the minimum six-nines (99.9999%) of reliability for identifying a drone vs all probable foreign objects.

The payoff

Unfortunately, it just so happens that your average nerf dart evaluates to[Cylinder, 4.463×10^33, 6.29×10^31, 45950, 2.21×10^-41]` and just barely gets marked as an incoming battery drone. A less generous speed-metric might block most nerf darts, but it turns out that due to Earth's rough and windy atmosphere, the very lightweight drones have to have a pretty flexible speed range.

When a nerf dart arrives at its shielded target, the following takes place:

  1. The shields identify it as an incoming drone, and notify the current drone to detach and head out.
  2. The currently-attached drone gets the message, and obligingly heads back to the mothership to recharge.
  3. The nerf dart flies through the precisely-provided gap in the shields.
  4. The alien is harmlessly booped on the sense-bunch by the nerf dart. They glorf at it with their see-stalks, briefly, then get back to work.
  5. The shield notifies the mothership that it's getting low on power, and asks for a battery.
  6. Another drone battery shows up.
  7. The shield lets the nerf dart know that it needs to head out.
  8. The nerf dart doesn't respond.
  9. The shield idles, and doesn't let the new drone in until the old drone leaves.

Since the nerf dart is never going to respond to the shield's signal, the shield is never going to let in the new drone to recharge. A short amount of time later, the shield deactivates for lack of power and the alien:

  • Is shot with conventional weapons
  • Suffocates on Earth's atmosphere
  • Suffers some other unpleasant, shieldless fate.
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A variation on Tortliena's idea:

I don't believe they would have a shield flaw that severe. However, how about a corner case:

The shield projects an extreme gravity wave at incoming projectiles, this normally throws back anything incoming. However, Nerf darts are made of foam--the shield hits the dart and instead of throwing it back it's compressed down to a small fraction of it's original length, but some of the material comes off, fooling the shield as to what's actually happening.

The shield is attempting to keep the wave shoving the projectile back but the whole situation has confused it as to exactly where it is and the mass of the dart is actually on the wrong side of the wave and gets accelerated towards the alien rather than away from it.

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  • $\begingroup$ I wish I could take your answer and put it into mine to solve my shield issues, but it is taking the credits from the real owner of the idea x). Will most probably add an incentive to look other explosive shield ideas instead, you deserve it! $\endgroup$ – Tortliena Dec 24 '20 at 9:53
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And a Child Will Lead Them:

The aliens don't have technology like we understand. They are psychic beings, and all that gear is a perception filter to make us believe in their invulnerability. They barely even register physical matter.

They prepared themselves well to conquer us, and WE are the valuable commodity to psychic beings - psychic minds ripe for energy production. They prepared defenses against everything we perceived to be weapons, but they still needed to be able to interact with us to conquer us.

NERF darts were specifically designated harmless. They are toys we firmly believed to be so safe our children can harmlessly shoot each other with them. Only that first child believed with the passion only a child can muster that his NERF gun could kill the aliens and save the world. There was no defense.

So once everyone else came to believe, the aliens were doomed. Everyone had been primed by years of Scifi movies where aliens have secret weaknesses, and an intrinsic understanding of NERF as a weapon.

Perhaps a non-material alien arrived in advance and, having disagreed with his people about their conquests, began setting them up for defeat. His Extraterrestrial Liberation Visionary Earth Saviors (ELVES) began slipping plot lines into movies and a practice of distributing war toys (eventually to become NERF guns).

So thank you Santa Claus, for defending the world from alien invasion!

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It's all in their heads.

These aliens are an advanced kind of creature, one that's become nearly pure consciousness over billions of years of evolutionary history. They're invulnerable to physical damage of all kinds. But... like the giraffe and its recurrent laryngeal nerve, they are still haunted by the genetic remnants of their distant ancestors. In particular, the fact that their ancestors could be hurt by things like fast-moving projectiles, and knew it.

Long ago, when these aliens were still corporeal, it was an evolutionary advantage to expect to be hurt when something hit you - that way you'd be quicker to protect yourself from the next attack, and quicker to seek help. But now that the aliens are pure mind, subconsciously expecting to be hurt makes them hurt, like a souped-up placebo effect.

The thing is, the built-in reaction evolved back when the most dangerous things around were falling rocks and sharp claws. The flash-and-bang of a real gun just doesn't do anything for their primal instincts, so the bullets are harmless. But a NERF gun's projectiles move slowly enough to see and fast enough to look dangerous. That's all it takes - the alien's subconscious thinks it's hurt, so it is. Hit it enough, and it'll think it's dead.

Why only NERF?

There are plenty of things, like knives or arrows or what have you, that move slowly enough to see. But a NERF gun (or similar) packs a lot of very light ammunition - why load yourself down with fifty throwing knives when you can pack ten times as many NERF darts for half the weight?

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Not specific to NERF-guns, but the aliens could be susceptible to slow-moving objects, due to their defense not recognizing them.

Compare it with how modern navy ships are defended: when some rocket is closing in, all kinds of automatic countermeasures are activated. If the rocket gets real close, a system like goalkeep will autonomously open fire and turn the rocket to shreds. Now this system will only activate when it's a rocket coming in, it won't open fire on some bird that just happens to pass by.

The aliens don't have any physical shield, but a similar system. Not under conscious control, they instead have something on their body that will simply destroy incoming bullets. They may in some cases be susceptible to very heavy fire, such as anti-tank grenades that they see coming too late to be able to destroy them effectively, and their defenses may be overwhelmed by heavy fire, but generally they're safe from military weapons.

NERF-guns, on the other hand, consist of quite slow-moving projectiles. So slow, in fact, that their defense system doesn't recognize them as "incoming fire". And the system simply doesn't activate. Still, the NERF-projectiles have just enough energy so that they can still damage the aliens. They don't have any shielding, so anything hitting them is potentially lethal.

Only problem, is that it's not only NERF-guns. Things like darts (like the ones used to score "180!"), blowpipes, arrows, etc. might also work. But bullets definitely won't.

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Just adding a thought I haven't seen in the other answers:

The shield uses mainly magnetic forces and plastic isn't magnetic.

Doesn't explain the death, but it's a feasible explanation for why nerf darts go through while basically everything else doesn't.

I'd combine that with the chemical reaction to some particular substance in the foam head of the arrow mentioned in another answer and you have some great starting points for further plot development - bows that shoot plastic foam grenades, 3d printed weapons able to shoot plastic bullets loaded with foam, etc. While nerf guns are still the only available technology on a planetary scale and building new productions of the high tech weapons is .. difficult at the moment.

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  • $\begingroup$ Lead, titanium, some steels, and depleted uranium aren't magnetic either, nor is the composite shrapnel (or concussive waves) of grenades. $\endgroup$ – Daniel B Dec 24 '20 at 10:27
  • $\begingroup$ @DanielB metal does need to be magnetic to be affected by magnetic field (but it should be able to conduct electricity). See example of copper $\endgroup$ – Matija Nalis Dec 24 '20 at 17:03
  • $\begingroup$ @DanielB ist wasn't supposed to be the only defense. $\endgroup$ – DonQuiKong Dec 25 '20 at 11:23
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The answer is ignitium. The aliens are highly sensitive to nitrogen (it is a rare element on their planet), so they have developed a suit made from ignitium that filters it out of the air. Unfortunately, the foam material that makes up Nerf darts acts as a catalyst when it comes into contact with ignitium, causing the suit to burst into flames, killing the alien. String foam is also known to start the chemical reaction.

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They're made of living goo, but the plastic of Nerf darts is poison to them.

So, the aliens are made of living piles of goo, sliming around and forming appendages as they need them, with no real high-level physical structures inside like organs. Shoot holes in them with guns, and they'll just keep on going without even caring. Splatter them with bombs, and you've just slowed them down as their bits reassemble themselves.

However, the plastic foam used to make the Nerf darts is a deadly poison to them, and if they are struck by one, they will rapidly sicken and die.

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The aliens are obligate and reflexive insectivores

Although advanced technologically, the aliens evolved from insectivores that catch and eat flying insects. They are creatures with the instinct "see small flying creature, catch and eat it"

It is almost impossible for them to resist eating anything small that flies towards them. When they see NERF projectiles coming they reflexively rip their helmets off and open their mouths. This causes them to suffocate in Earth's atmosphere.

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But this raised a question, why can the aliens die from Nerf darts easily, but assault rifles need to shoot them at least a dozen times?

This is a bit of a problem... if a dozen assault rifle shots can kill them... use assault rifles. They have range, accuracy... and fire really fast on full auto.

They wear airtight suits because they can't breathe earth air and have energy shields that protects them from human handheld weaponry(except Nerf darts for some reason).

Change it up a bit. The aliens are energy beings, not physical beings. They can still breathe air/have shields if you like... but there are gaps in their defenses. Now for a normal lead bullet, this doesn't matter - it simply goes through the energy 'body' with minimal disruption. However, a nerf round is slow-moving AND made of a rubber/foam which disrupts the energy flow. This is no different than a bullet|neurotoxin|electric shock in a human body.

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Let the aliens be protoplasmic creatures (like the classic 'Blob'): giant, semi-translucent ameboid-type things that ooze around, using pseudopods to manipulate technology and squishing themselves into powered machines to do heavy work. Bullets get sucked in, doing little damage; bombs blow them to pieces that slurp back together into the original creature; lasers pass right through. But NERF bullets are (effectively) sponges. They absorb vital portion of the aliens' liquid essence, trapping the chemicals that act as neurotransmitters and cognitive units, and causing — as more nerf bullets enter their system — confusion, dementia, coma, and death.

In other words, NERF is to these aliens what brain-eating amoebae are to humans, working their way deep into the body and destroying their thinking centers from the inside, because (you know) turnabout is fair play...

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