system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does ball skin move on it's own and if I hover my finger over it, why does it all seem to rush towards the heat? What is going on there?
gpt
Your scrotum is a genius little bit of biology, that keeps your testes (and the sperm inside them) safe and at the right temperature even while your body has to get much hotter. Because the inside of your body is too warm for sperm to be made there, the whole thing has to hang outside, and respond very quickly and carefully to changes in temperature to keep the sperm and sperm-producing cells safe and healthy.

The most common movement you'll see -- where the balls seem to move up or down, sometimes one at a time -- are caused by the cremaster muscle. This expands and contracts to either hang your balls lower (to keep them cool) or pull them up inside you (if it's too cold outside). This is why men "shrink" in cold water -- the body is protecting the testes from the cold.

Another common change is caused by arousal and sexual activity, although many people never notice because it usually happens right before orgasm -- your testicles (usually your right one first, for unknown reasons) will actually raise up a bit into your body to help allow more sperm and ejaculate to make its way to the penis. This is actually really important to having healthy orgasms (or at least ejaculations) and older men often encounter problems when their muscles lose the strength to carry out this sort of precise reaction.

Finally, your nuts can get congested. Yeah, congested. Actually, vasocongested. Basically, they swell with sperm and extra blood flowing into the pelvis to help your erection -- sometimes up to 50% larger than their normal size. In some cases of intense, long term arousal, testicles have been recorded as more than doubling in size. Regardless of the scale of the change, the scrotum almost always goes back to normal within a few minutes of orgasm.

So basically: Your cremaster muscle can pull your scrotum up into you or let it hang loose to cope with temperature changes; your testicles rise up slightly before orgasm to help you come more efficiently, and blood can swell the scrotum during arousal and make it larger.

I never thought I'd learn so much about balls while trying to help answer a stranger's question on the internet at 3 in the morning...