system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a nuclear bomb work?
gpt
Some elements are unstable. 

Normal Uranium (U238) is a little unstable. It's angrier friend Uranium 235 is *quite* unstable. It doesn't take much extra energy to make U235 atoms split. When a U235 splits (fissions) it turns into 2 smaller atoms, a few high speed neutrons, and a bunch of energy. 

When you stuff a whole bunch of U235 atoms together in a small space those neutrons have nowhere to go but into other U235 atoms, which causes them to split and release more neutrons, which in turn hit more atoms, that release more neutrons, etc.

If you spread your Uranium out a bit and absorb some of those neutrons with stable materials (like graphite or water) you moderate the reaction and you get a very hot bunch of Uranium you can use to make electricity. 

If you smush all the Uranium together and don't moderate the reaction you get **A LOT** of energy. Like, enough energy to create a city-sized fireball. 

So simple nuclear weapons are basically just a Uranium squisher. The simplest kind is the gun-type bomb. It's basically a tube with a big chunk of U235 at one end, and a smaller chunk of U235 and some explosives at the other. Separated, the two chunks of Uranium are more or less safe. Then you trigger the bomb and the explosives blast one chunk into the other. Now you  have what is known as a super-critical mass, basically too much unstable Uranium in too small a space, and blammo.

The more complicated type of fission bomb uses a carefully timed series of explosions to turn a hollow Plutonium sphere into very small Plutonium ball. Same result though, too much Plutonium in a small space goes kaboom.