system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is nuclear fallout/radiation and why does it make us so sick?
gpt
All the stuff in the universe is made of up of little tiny pieces called atoms.  Atoms are like little balls of energy wrapped up, and over time they will "pop" and the energy will explode out.  Many atoms are "stable" which means that it takes a very long time for the balls to explode, longer than the earth has existed even.  But some are "unstable" and explode sooner.

This happens all the time all over the universe, even right now in this room.  But because atoms are so tiny you never notice the tiny little explosions.  These little explosions can hurt you, a little tiny bit, like a very tiny bruise or cut.  But just like your body can heal a cut or a bruise over time, it can heal the tiny cuts from these tiny explosions.  

Some clever people found out how to gather up a lot of "unstable" atoms in one place, and they can use the exploding energy for lots of different things like Nuclear Power, Radiation Therapy, or even make a Bomb with it. Things like Fallout happen because a lot of unstable atoms were gathered together in one place, and these cause a *lot* of tiny cuts and bruises if you get too close to it.  More than your body is used to, and that's why you can get sick from it, it's too much for your body to fix all at once.