system
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human
[Marvel] Does Reed Richards change his mass, density, etc. when he stretches?

This is really a question for any elastic superpower, but I thought if we had to draw off of one he’d be the most popular and have the most information on his abilities.

When an elastic character, like Reed, grows his fist to warhammer size, is it technically the same weight, just less dense since its cells have spread out to a different shape? It’s expanded, but does it actually hurt more to get hit with?

And when he makes himself ultathin to enter a crack or vent, for example, is his mass decreasing or is it just compressed? Is he getting denser?

Basically, how do his properties change with his growing and shrinking?
gpt
He changes density, not mass; he becomes physically weaker as he elongates, and stretching becomes physically painful at around 1,500 feet.

When he forms melee weapons, he's shifting his mass into the fist; the weapon is larger and denser than normal flesh, but the rest of his body is correspondingly less dense. He might also use the elastic properties of his flesh to enhance the speed of his attacks, essentially slingshotting the weapon at the target.

When he fits into cracks and such, he isn't really changing density, just distribution. A sheet of paper has the same density laying flat or crumpled up, and Reed has the same density in his normal form or when squeezing through a pipe.

Plastic Man, for what it's worth, works a bit differently. There seems to be no limit to how far he can stretch or how dense he can become, which means there is some sort of matter manipulation happening, not just a change in density. Batman has said that Plastic Man is actually the most dangerous meta-human he's ever encountered.