system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes things bouncy?
gpt
Oooh, awesome question!

There's a few things, but first, what does it mean for something to be bouncy?
It means that when an object collides with something else (for now, let's imagine a perfectly rigid wall), it leaves with the a lot of the same kinetic energy it had before the collision. A "perfectly elastic" (100% bouncy) is one where the ball has the same kinetic energy it had before and after the collision. 
There's a few ways to lose energy, but mainly they are: sound, heat, and deformation. 

So when something is super bouncy, it rebounds really well, and doesn't lose a lot of energy doing so. The big item on that list is deformation. Throw a rubber ball against a wall, and it bounces right back. But also, it's still totally ball shaped afterwards. Contrast that with a ball of Play-Doh. It won't bounce, and it will flatten a lot. These things are related--the play doh can't voice because almost all of its original energy went into changing shape (this consumes energy because it has to break a lot of molecular bonds and move a lot of material around). 

So some things are not bouncy because they're really squishy and change shape easily, like play doh. Other things aren't bouncy, but are still pretty hard. Metal doesn't bounce much because most metal is made up of tiny crystals. When you deform a metal, these tiny crystals move around a lot, changing the overall shape and using up that rebound energy. 

Some things are bouncy though, like glass. Glass is surprisingly bouncy. The reason it is is because it doesn't have a crystal structure, so it can't really change shape the same way metal can. Either the energy goes right back out as bouncing, or it goes into shattering the whole object. This tendency to shatter is part of why we don't normally think of glass as bouncy. 

Now rubber, that's some cool stuff, and probably what everyone thinks of as bouncy. One thing it has going for it is that its molecules are really long and intertwined. These molecules fold and coil and do all sorts of things, but when stressed, they straighten out and elongate. Basically, rubber is made up of molecular springs. This works differently from the grains of a metal that all shift around permanently. As long as you don't break the chains, the molecules in rubber will bounce back to a similar shape after being stretched or compressed, so the energy from the impact gets released and the material recovers its shape. This is another part of why we think of rubber as bouncy, because it's really good at distributing stress. So when you throw a rubber ball against a wall, the point that hits the wall doesn't break, it instead spreads the load around.