system
Given the following post, respond as if you're commenting on Reddit:
human
ELI 7: Radiation 

Not the heat version.  The kind that's interesting to 7 year olds.  The kind that makes mutants and is the result of nuclear explosions.  I have a 7 year old who just asked me this and I figured "This is a job for Reddit!"
gpt
All matter is made up of atoms. Sometimes, we find atoms that are 'unstable', and will suddenly 'pop'.

When it does, a little 'bit' of the atom flies off at high speed. That's radiation.

The biggest pieces are like cannonballs. They're big, heavy, and can't go very far - a few inches at most.  When they hit something though, they do a lot of damage. We call these 'alpha' particles.

The next size down is like a bullet. It's a lot lighter than the cannonball, and can go a lot further, but if it hits something the damage is less. We call these 'beta' particles.

The next size down is more like a laser, because it doesn't weigh anything at all! It's actually just like a laser, only it's a kind of light you can't see. It goes really far, even through *outer space*, and most likely will pass right through you. But it can 'burn' you like a laser can, so we have to be careful. These are called 'gamma' rays.

The fourth kind is like a grenade. It's a little bigger than a bullet and smaller than a cannonball, and doesn't really hurt whatever it hits....yet. But like a grenade, it causes another little 'explosion', and another atom 'pops'. We call these 'neutrons'.

Here's a neat thing for you; if one of the 'grenades' or neutrons makes another atom 'pop', and that atom also releases a 'grenade' when it pops, we can try to string them together so they keep popping more atoms. This is how nuclear reactors work.