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A lot of people are probably familiar with this sort of handwavium in fiction when it's asked where a shapeshifter's extra mass comes from. Most choose to ignore that question. In works that pretend to be slightly more credible, they simply state that no extra mass is being created at all and that the shapeshifter is in fact merely grossly obese. Or at least they would be, were they not using their power to simply make themselves look like a 200lb man who actually weighs 500lbs. Presumably this is done by eliminating a lot of the empty space either in the human body or between the molecules themselves.

But this begs the question. How much mass can you actually handwave away like this before it becomes really obvious that this 150lb woman is actually a 700lb shapeshifter in disguise? Density is still a thing, and at some point the surface area to density ratio of all this compacted flesh and biomass is going to make the shapeshifter's legs sink into the very ground they walk on, no matter how careful they are to conceal their true weight.

When would this happen? What's the realistic limit to how much mass a shapeshifter could conceal in a humanoid frame before the extra weight becomes obvious? I'm assuming it would have to be greater than 500lbs but I'm not sure.

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    $\begingroup$ Take 2 similar balloons partially filled water and boils one of them, despite no significant difference in their total masses but the one containing vapour becomes less dense than the surrounding air... just a proof of concept not necessary accurate.😃 $\endgroup$ – user6760 Jan 14 '17 at 7:23
  • $\begingroup$ Are you content with a plausible shapeshifter organism ? One I could build would be lighter than pretended, so something looking like 300 lbs would be actually e.g. 100 lbs heavy. $\endgroup$ – Thorsten S. Jan 15 '17 at 20:05
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Due to my own weight, I was really interested in shoes. And this can be a good way to see this.

High heels break way too easily if the user weighs more than 100kg / 220lbs. Actually, most soles are designed around this maximum weight, but on flat soles, like sneakers, it would be much harder to actually notice damage. So, your shapeshifter would wear flat shoes, and change them much more often, or would wear shoes from the shops for heavy people (like fat people shops, shops for sportsmen etc).

Other things are ladders and old stairs. Your typical ladder has a maximum load of 150kg / 330 lbs. You will probably not see your 700lbs shapeshifter using ladders, and wooden stairs will creak under it.

And then, of course, you can take it swimming. If he is walking on the bottom instead of swimming, here you have your shapeshifter.

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The easiest way to switch mass to volume is fat vs. muscles.

You've seen those pictures where 5lbs of fat was the size of a forearm where 5 lb of muscle where size of a fist. So the obvious way would be to hide that extra mass in super hard biceps and abs. And by hard I mean deadlift plates hard.

In the greatest battles between pinky and nightstand shapeshifter would kick that nightstand into daylight. That's how you would spot it, the energy needed to move that weight would end up on objects coming in contact with the shapeshifter. Broken handles, buttons pushed into the coffee machine.

You ever tried to pull doors that need pushing? Shapeshifter would pull them out from hinges. Him putting his hand on a table would be equal to you smashing your hand on that table.

If he was aware of this you would notice that he avoids elevators with people in them as his weight could trigger an alarm. They wouldn't sit casually on things like tables, sofas or wooden chairs. They may even not sit at all not being sure if the chair would support that weight. They would not tap their fingers on surfaces because that could leave dents.

They would act as a 700lb human. I'm not sure about the puffing and sweating (we're talking alien here). They would use other people to do things for them (like opening doors, windows, moving objects) and avoid interactions of any kind. So no people touching them, no sports.

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