It's a fairly common legend that trolls turn to stone when exposed to daylight, and I was thinking about using such a creature in a story. I'm searching for a plausible way of justifying such a weakness - there's nothing similar existing in nature that I know about.

For background; I'm visualising these trolls as primitive, ape-like creatures that can grow to very large sizes. They are nocturnal / subterranean animals, and the sun is bane to them. They can move around on the surface at night, but in the daytime they have to take shelter under bridges or in caves.

Direct sunlight is their big weakness. These trolls are like lobsters - they are biologically immortal and they will continue to grow for as long they live. The trolls keep on growing and getting larger until they can't support their own mass, or until they can't hide from the sunlight any more.

When they die in the sunlight, their skin calcifies and they become rock. It's a slow and painful death; their skin turns to stone from the outwards inwards, until they finally crack apart.

The very largest trolls might become mountains. In this world, there are lots of strange rock formations that have been left behind by dead trolls.

It's possible that young trolls have resistance to sunlight and can operate in the day, but as they get older they lose this resistance. Young trolls are born in droves, but very few of them reach large sizes.

So; biologically speaking, what could justify this fatal trait of trolls? I want to avoid just using magic as a reason, and I'm more looking for broadly feasible suggestions about how and why trolls might develop like this. Is there any deficiency, medical or evolutionary reason on why would sunburn would cause troll's skin to calcify?

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    $\begingroup$ What's the justification? Tradition. That's how trolls work in stories, so we accept it. IOW, Bellisario's Maxim: Don't examine this too closely. $\endgroup$
    – RonJohn
    Commented Aug 22, 2018 at 21:11
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    $\begingroup$ According to Tolkein "for trolls... must be underground before dawn, or they go back to the stuff of the mountains they are made of, and never move again....". The master knew it was better not to explain. $\endgroup$
    – rghome
    Commented Aug 23, 2018 at 11:43
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    $\begingroup$ Terry Pratchett has a novel solution for his trolls. Trolls are always stone, they are living stone and possess a brain of impure silicon and thus rely on cold to operate efficiently, in the warmer climates where humans live the sudden heating caused by sunrise is enough to knock them out temporarily. Normally trolls live in the arctic or high mountains where this is not an issue. This also explains why dwarves and trolls don't get along, dwarves break rocks to get valuable minerals and trolls are rocks full of valuable minerals that look just like boulders when asleep. $\endgroup$
    – John
    Commented Aug 23, 2018 at 14:30
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    $\begingroup$ Trolls live under bridges and eat billygoats!!! (Unless you're young and were only ever told the politically correct version. $\endgroup$
    – RonJohn
    Commented Aug 23, 2018 at 14:57
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    $\begingroup$ I'd recommend finding the Blog of Greg Weisman (it's called Station 8). Weisman created the Gargoyles Cartoon in the 90s and the titular creatures had the same problem as your trolls, and for purely biological reasons. I forget the specific biological process he cited, but I know he was making references to biology. I can provide a link later. $\endgroup$
    – hszmv
    Commented Aug 23, 2018 at 15:22

12 Answers 12


Their skin contains a set of proteins that will bind to each other and harden when exposed to certain wavelengths, much like dental composite polymers:

Composites are placed while still in a soft, dough-like state, but when exposed to light of a certain blue wavelength (typically 470 nm), they polymerize and harden into the solid filling (for more information, see Light activated resin).

And from the link in the quote:

A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light.

So they are not becoming actual rocks. Their skin are becoming a very hard organic material that has much the same properties as most kinds of rocks.

Kinda like porphyria, but you become a statue rather than getting 3rd degree burns from sunlight.

For more effect, rather than hardening on the skin, the proteins can break loose from it when exposed to sunlight. They fall in the bloosdtream and then harden within other tissues as well as the skin itself.

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    $\begingroup$ If that's the case, wouldn't they just die because they are sealed inside a solid wall of stone, formerly their own skin / flesh? This would make e.g. breathing impossible an lead to a death by suffocating. $\endgroup$
    – Tobias F.
    Commented Aug 23, 2018 at 7:04
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    $\begingroup$ You should add that the polymerization of the outer skin might trigger a chain reaction that does the same to the inner muscle and viscera. $\endgroup$ Commented Aug 23, 2018 at 12:30
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    $\begingroup$ sciencenews.org/article/… May I also suggest waterbears, which turn to a crystal like structure without water. But they also turn back when water comes back. $\endgroup$
    – WolvesEyes
    Commented Aug 23, 2018 at 18:03
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    $\begingroup$ Nice; also has an interesting consequence if there are some smarter trolls (or more technologically advanced people controlling and such): if they can harness the right light in small, targeted amounts, they could make themselves armor plating. $\endgroup$
    – Delioth
    Commented Aug 23, 2018 at 19:48
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    $\begingroup$ @Carbine if these trolls polymerized, even if it were reversible, I'm pretty sure their biology is too complex for their de-polymerized hearts, lungs, and neurons to all just spontaneously fire up again (especially those who've been long dead). Alternatively if "normal" trolls are somewhat sentient and can communicate/negotiate/etc on even a rudimentary basis, then perhaps "revived" trolls could simply be mindless raging beasts, their normal cognitive function permanently deteriorated by the process. I'd imagine they'd die again in the next sunrise though, lacking intelligence to avoid it. $\endgroup$
    – Doktor J
    Commented Aug 23, 2018 at 20:44

Another classic trait of trolls, is a tremendous regenerative ability. Sometimes to the point where only fire can kill them.

So I'd suggest trolls create a chemical in their body that boosts their regenerative abilities, and any damage will increase the levels of this chemical. Sadly (for the troll) this chemical reacts with UV light from the sun, and hardens. Creating the resemblance of turning to stone.

That could be an explanation for why only sunlight, and not other kinds of light (fires usually don't emit UV light) it also explains why they haven't evolved out of such a vulnerability, as they would loose their regenerative ability.

It can be adjusted from only their skin turning to stone, and at night they can break free (loosing their skin, but it'll regenerate almost immediately) all the way to turning completely to stone (UV light can penetrate pretty deep, that's why we use sunscreen to not get cancer)

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    $\begingroup$ I really like this answer because it helps explain why the weakness wasn't eliminated by evolution. Such an extreme weakness to sunlight would be such a crazy disadvantage that this really does need to be explained. $\endgroup$
    – Nicholas
    Commented Aug 23, 2018 at 15:06
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    $\begingroup$ A cool twist on this could be that the regenerative enzymes are light activated. At night, or in dark caves, there is very little (blue end) light and so the enzyme works normally I.e. absurdly well. In the sun however it goes into overdrive and repairs the trolls body even where not necessary. It has different potency for different tissues and it just so happens that it turns the troll into its own bones. There are some details to be considered there: e.g. the source of calcium for the bones. Also; this would explain younger trolls immunity - less effective regeneration enzyme. $\endgroup$ Commented Aug 23, 2018 at 16:03

The frozen troll is the second stage in a two part lifecycle.

Part 1:

The troll is mobile, and goes around eating minerals to store in its body, and killing and eating animals (or hobbits) to sustain itself. It gathers and stores sperm from other trolls to enable it to reproduce in its sessile stage. The mobile troll focuses on growing, eating and generally causing havoc. It also carries dormant algae in its skin.

Part 2:

The troll is exposed to sunlight, which activates the algae on its skin. This triggers a biological reaction in the troll which causes it to rapidly harden into an armored algae fortress, by precipitating all the minerals it stored by eating rocks. The algae now live and thrive off sunlight and stored minerals, while enjoying the protection from herbivores afforded by the troll's frozen body. They produce lots of sugar, which is stored deep inside the troll. Most of the troll's cells die; however, the reproductive system is sustained by the algae. The troll-rock then can exist indefinitely, producing a steady stream of tiny trolls, which gather algae spores on the way out, then grow up and continue the cycle.

Becoming a troll rock is incredibly unpleasant, as the brain of the troll survives while frozen in place until it starves as resources are diverted to reproducing. For this reason, trolls do their best to avoid getting frozen, but eventually they slip up and continue the cycle.

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    $\begingroup$ This is creative, love it! $\endgroup$
    – Korthalion
    Commented Aug 24, 2018 at 12:23
  • $\begingroup$ This would make for a fascinating culture. Imagine if the only way to repopulate was to die a horrible slow death encased in rock and algae. How would the society look at that? Would they nobilize the act as a necessary sacrifice to be commended. Would they encourage the elderly to go out into the light to stop being a drain on resources and to spawn more trolls? Would trolls try to purify themselves of the algae to avoid the negative side effects of sunlight, but then effectively become sterile? Moral quandaries a plenty. $\endgroup$ Commented Nov 22, 2020 at 21:23
  • $\begingroup$ My only critical question would be why would a troll consume minerals if they know that is what leads to the horrible immobile death? Is it because it is somehow necessary for the life cycle? $\endgroup$ Commented Nov 22, 2020 at 21:23

Gypsum trolls

Trolls really are softies (literally) on the inside. As soon as a baby is capable of moving, troll moms pound gypsum with their clubs, heat it up, and roll them around in the resulting plaster of Paris. As they grow older, the old layers slowly crack, and more plaster is rolled on, giving the trolls a tough outer shell. As the plaster sets, it also cools, thus helping the massive creatures cool down. This is why the biggest baddest trolls spend all their time wallowing in plaster pools.

Now, this isn't a problem -- trolls live in moist caves, by continuing to move and keeping the joints wet, they ensure that the plaster sets slowly, and only sets in desireable parts of the body. However, once they go outside in the sunlight, their eyes (so perfectly adapted to the dark) get blinded; making it hard to find shelter before the plaster sets into all their joints, leaving them immobile, dying a slow death.

For added fun: gypsum also forms beautiful crystals. Trolls can "wear" these crystals either by sticking them onto still wet plaster or by encouraging crystal growth on their bodies. Hardened trolls look distinctive in the landscape: blinding white structures adorned with glittering multi-colored crystals.

Giant gypsum Crystals in Naica Mine Giant gypsum Crystals in Naica Mine, Mexico (See person for scale). Source: Alexander Van Driessche

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    $\begingroup$ I could also see this working with cements of some kind. $\endgroup$ Commented Aug 23, 2018 at 9:39
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    $\begingroup$ The chemical process that hardens cement will work under most conditions (even under water), once cement has become wet the process starts and is irreversible, that makes it less suitable in this context. cement.org/cement-concrete-applications/… $\endgroup$
    – Douwe
    Commented Aug 24, 2018 at 9:02

Let's assume that trolls are unusual creatures, in that they consume and metabolize rocks, allowing them to access an abundant, yet generally low-competition material, as a food source.

This is what allows them to grow to huge sizes, as they gradually assimilate more and more of their mountainous surroundings.

However, it does come with a disadvantage: the chemical processes that allow them to integrate rock material in their organisms are expressed throughout the body and must continuously remain active in order to preserve tissue viability. Since trollkind has evolved deep underground and only in recent millennia has emerged to the surface through cave systems, their special metabolism is particularly sensitive to UV light. Once exposed to UV rays, even at low levels, a cascade reaction is triggered starting from their skin, turning the chemicals responsible with tissue viability into an inactive form.

Without these chemical processes, rock material reverts to its passive, inorganic state and the troll quickly and painfully turns to stone, from the outside in.


Specific photosynthesis

Trolls can continue to grow for as long as they live…

I know another sentient being that have the same property: Trees.

What if trolls were closer to trees than to animals?

Yes, trolls evolved a lot since that time where they had roots instead of foots but they still share a lot with their ancestors… Especially photosynthesis!

But trolls have a particular sort of photosynthesis, the oxygenic photosynthesis:

CO2 + H2O + photons light energy → [CH2O] + O2

This reaction is pretty useful for them as they are living in caves where the CO2 concentration is very high! That’s the reason why this photosynthesis has to be very efficient: without it, trolls would suffocate in their lairs.

But… There is not light in their cave so how is this possible? In fact, there is light in these caves: from fireflies and glowing mushrooms… And trolls have evolved in such a way that this tiny amount of light is enough for them to produce a lot of O2.

So… When a troll is exposed to direct sunlight, the amount of light it takes is insanely huge for him and its photosynthesis reaction gets instantly mad!!!

Problem : This reaction consumes water! Therefore, the troll is desiccated in a matter of minutes and the remaining corps just looks like a sort of mummy or a rock…

By the way, this tree-affiliation and the photosynthesis would explain their green skin and their mossy look...

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    $\begingroup$ Nice, although this is one heck of a pseudoscience explanation... $\endgroup$ Commented Aug 23, 2018 at 15:07
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    $\begingroup$ Trolls are Stone Ents? $\endgroup$
    – Arluin
    Commented Aug 23, 2018 at 18:13
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    $\begingroup$ @Arluin Tolkien's trolls were made "in mockery of Ents" $\endgroup$
    – Wlerin
    Commented Aug 26, 2018 at 5:03
  • $\begingroup$ @leftaroundabout When dealing with immortal mountain-sized-trolls, pseudoscience is maybe more adapted than hard-science... ;) $\endgroup$ Commented Aug 27, 2018 at 9:06
  • $\begingroup$ tvtropes.org/pmwiki/pmwiki.php/Main/InsaneTrollLogic $\endgroup$ Commented Aug 27, 2018 at 9:08

One aspect of common sunburns is the body's inflammatory process. Perhaps the biology of your trolls is such that their inflammation redirects calcium from their bones to the surface of their skin, where it merges with their skin cells to calcify it. Then, like a hemophiliac response to a bruise, hemorrhage or cut, the process continues in a runaway fashion until their whole bodies are turned to "stone" (or bone).

Your trolls would need a large supply of calcium for this to make sense, so they should have especially large, thick, and dense bones.

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    $\begingroup$ Why would inflammation redirect calcium? $\endgroup$
    – RonJohn
    Commented Aug 22, 2018 at 21:06
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    $\begingroup$ @RonJohn why would Trolls exist? $\endgroup$
    – Hobbamok
    Commented Aug 23, 2018 at 8:33
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    $\begingroup$ @RonJohn It's a biological problem that was never selected against evolutionarily. Similar to our optic nerve going the wrong direction (into our eyeballs) and then out the back again, causing a blind spot in each eye. Or strobe lights causing some people to have uncontrollable seizures. $\endgroup$ Commented Aug 23, 2018 at 14:51
  • $\begingroup$ But we know (or have a reasonable explanation) why the optic nerve goes the wrong way. What's the reasonable explanation (that's the justification) that inflammation redirects calcium? $\endgroup$
    – RonJohn
    Commented Aug 23, 2018 at 14:54
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    $\begingroup$ @RonJohn I don't have an answer for that. I don't know biology or evolution well enough. As I said, my best explanation was that it was a mutation that never got weeded out by evolution, since it doesn't interfere with this nocturnal species' ability to reproduce. $\endgroup$ Commented Aug 23, 2018 at 16:31

You might also consider why they would react to light by turning to stone. My suggestion would be as a defense mechanism against some other type of creature that produces light. The troll's skin turns to stone as a defense against that creature. After a few nights, the stone skin would slough off and allow the new skin under it to grow out.

Assuming the attacking creature uses a reddish, long wavelength, the problem with sunlight might be that it includes a more penetrating short wavelength ultraviolet light. This penetrates into the interior of the troll, turning the organs and muscles to stone. Trolls might be naturally somewhat translucent in strong light, although most people never get a chance to see it.

An alternative would reverse things. Instead of light being the issue, the real issue is heat. It's the heat from the sun that turns the troll to stone. As more and more heat is transferred, it effectively cooks the troll from the outside in. Heat of course can be conducted through stone, unlike light. This of course would also allow adventurers to set trolls on fire to turn them to stone.

I'll leave it to others to explain how the conversion to stone occurs. I'm just trying to explain why it would evolve.

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    $\begingroup$ I would avoid the defense mechanism explanation. Turning to stone isn't beneficial to the troll at all; there's no way to turn back. Sunlight is a death sentence. Rather, it's more like a fatal flaw in their DNA. Other species have self-destructive coding that makes them grow old, but trolls have self-destructive coding that turns them to stone. $\endgroup$
    – user54563
    Commented Aug 23, 2018 at 8:12
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    $\begingroup$ @Wyvern: actually the described defense mechanism isn't against the sun (which emits also UV) but against an IR-emitting creature (bot the troll and the creature usually live underground). Stated that such a creature do exist, the thin IR-activated rock-skin is a good defense against IR overheating, rock is insulant. The issue is that the sun provides also UV, which are more penetrating and affecting also deeper layer. They evolved underground, therefore no big deal with the sunlight, they just avoid it as we avoid ocean floor. $\endgroup$
    – theGarz
    Commented Aug 23, 2018 at 9:32
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    $\begingroup$ Actually UV penetrates most materials less, so this might be more plausible if set up the other way around: the predator emits only a bit of UV, which merely hardens the outer shell. Whereas the sun also has a lot of visible light, and that reaches deep down and is still energetic enough to cause the hardening where it was never supposed to happen. $\endgroup$ Commented Aug 23, 2018 at 15:12

So; biologically speaking, what could justify this fatal trait of trolls?

I'm more looking for broadly feasible suggestions about why trolls might develop like this.

It's an accidental feature.

(I'm going to assume polymers hardening in the skin is what causes the solidification as per other posts, and add the possible reason the trolls have survived while still being sensitive to light.)

Originally trolls grew up in the earth, where sun would never reach. Trolls would die frequently when young, so they never got programmed to stop growing, even though this would be a major defect in a small space underground. Call it a side effect of high breeding potentials, they grow fast and loose. Sorry, anyways these small cave dwellers eventually get better at surviving longer, until some of them are large enough that they start to migrate more, looking for bigger passages. Some will get stuck and die, but others will find big places to live, and still others will reach the surface. Of course, as soon as the sun shines upon them their skin starts to harden, so they quickly learn to avoid it, and those who are better at avoiding it can even live on the surface. But that can also be a risk that potentially ends with them being turned to stone/hardening polymer/whatever. Point being, sometimes evolution is more of an art than a science, and not every creature is the best adapted for new environments.

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    $\begingroup$ This doesn't actually answer why "as soon as the sun shines upon them their skin starts to harden"... $\endgroup$
    – walrus
    Commented Aug 23, 2018 at 9:25
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    $\begingroup$ @walrus I would have given a polymer answer, but since that's already been done I decided to answer "why trolls might develop like this." Instead of the what makes this happen part of the question. The question is multiple parts. $\endgroup$ Commented Aug 23, 2018 at 9:36
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    $\begingroup$ @YElm The how it hardens is because photoreactive polymers, or a similar process that is assumed. I am purely addressing the "why" this creature exists. I could give an explanation as to why its skin has polymers inside it, but it's not going to be important for the story, and in the event some other process is chosen, my answer still gives the advantage of explaining where a creature that can die under sunlight would come from. None of the other answers really addressed this important question. $\endgroup$ Commented Aug 23, 2018 at 10:06
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    $\begingroup$ @walrus "So; biologically speaking, what could justify this fatal trait of trolls?" " I'm more looking for broadly feasible suggestions about why trolls might develop like this." Am... am I imagining this? $\endgroup$ Commented Aug 23, 2018 at 10:21
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    $\begingroup$ @walrus I kind of disagree with the one question only rule in cases like this, because the answers to both will be inherently linked. Well, I worked around the variable reasons the skin would turn hard and only answered how it happened without being an evolutionary burden, but in a lot of cases the how and why of evolution are linked together. Now I'm not saying all cases should be left as is, but a certain tolerance for questions that influence each others answers seems reasonable to me. $\endgroup$ Commented Aug 23, 2018 at 11:13

It doesn't need to be a natural feature in order for it to be non-magic. It could be the result of genetic engineering. In the distant past, trolls didn't have this feature. But, some trolls enslaved others to work underground in mines. The trolls that were slaves were genetically modified so that they calcified when exposed to sunlight. This limited their ability to rebel. Eventually, a surface catastrophe (e.g. asteroid impact) caused the surface trolls to become extinct. The former slaves emerged from the mines and found that they could get by just fine at night but would die if caught exposed to sunlight during the day. They have been that way ever since. Since they are well-adopted to a nocturnal life there hasn't been sufficient genetic pressure to remove this engineered trait.


Declare trolls to be immensely specialized to their lifestyle. Everything about them is fine tuned like a shark, so that they can reach unbounded sizes. Its not easy to design a creature that can function at utterly any size, so you're going to have to give up some flexibility in terms of the environments. Some environments are simply inhospitable to what's left after you focused on unbounded grown for that long.

So the trolls have a natural solution. If they find they are in any environment which is not suitable for troll life, they go into defense mode. They literally freeze in place, and wait for the environment to change. It's a similar response as we see in armadillos, which curl up into a ball in response to a predator and wait for the predator to go away.

For the most part, this is effective. If a proto-troll, earlier in their genetic history, were to get caught out in the sun, this freezing-stone-form effect would keep them safe until the sun went down and the environment was once again troll friendly. It was very useful from an evolutionary perspective.

Now what people don't know is that trolls are rather sensitive emotional creatures. They have developed a level of empathy which we can only dream of. And, as such, it is emotionally painful to lose contact with the outer layers of the skin as they enter stone form. As such, inner layers may also turn to stone, in an attempt to avoid the sadness from the loss of contact.

Old trolls eventually develop this empathy so greatly that when the outer layers of their skin freeze, they try to hold onto them so hard that their inner core eventually freezes as well. And this leaves you with a stone troll, forever marking the moment in time where they just couldn't let go.


You said you want to avoid just using magic as a reason, but maybe it would provide an interesting additional layer in your story if you gave the facts a mythic explanation as well (that perhaps your characters grapple with by contrast with the scientific explanation).

So here goes:

Trolls were not part of the rest of creation but were the work of a rebellious power, one opposed to the power responsible for the sun.

With normal creation, the sun has a purifying effect that causes the creatures to regenerate and the plants to thrive, and killing off pathogens: a system set in place during the original creative work.

But because trolls didn't come from that work, the sun's purifying function is against them. So, just like a virus or a bacteria can't survive under UV exposure, neither can a troll.


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