system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does radioactivity work

If radioactivity is a decaying atom.  What is left behind at sites like chernobyl.  What causes some atoms or isotopes of atoms to be radioactive while others are not?
gpt
Some atoms are just unstable and will eventually break apart. Imagine them like a bunch of legos chained together and being spun. Some configurations will allow a little piece to go flying off while others will stay together. 

At Chernobyl, Uranium atoms were being split in half in the nuclear reactors. When uranium splits in half, the two smaller atoms that remain (each being about half the size of U) are typically unstable (they have too many neutrons). So they convert neutrons to protons in what we call *beta decay.* This happens slowly and releases some energy as it goes. That energy can cause biological damage and is therefore dangerous (especially if ingested).