system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can a human float on their back in water if they relax, but sink when they tighten up there muscles. How does it change our buoyancy?
gpt
Well, the density of humans varies from one person to the next, but assuming that the human in question is an average person (slightly less dense than the water) then a little bit of that person's body is going to stick out of the water. 

When a person is on their back, the highest part of them is their nose and mouth. When a person is upright, the highest part of them is the top of their head.

Secondly, when a person is relaxed and spread out (as they are when they are floating on their back) they are more stable, because most of their body is close to to the horizontal point of neutral buoyancy. When a person is tensed and angular, they will bob more, because different parts of their body are at very different places in relation to that point of neutral buoyancy. (Compare rolling horizontally across a trampoline vs walking across a trampoline)