system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes plastic recyclable?

What is the difference between recyclable and non-recyclable plastic (materials)? Similarly, why are some plastic bags degradable while others aren't?
gpt
There are a lot of different ways to make plastic.  Even though it all sort of seems the same when we look at it or touch it, there are many different types.  These types of plastic are made of different "ingredients."  The most common two ingredients for making plastic are ethylene and propylene, which you might have learned about in science class.  These plastics are hard to recycle because when you melt them down to reuse them, they separate out into different layers.  When those layers cool, it weakens the plastic, making it much less useful.  So, if you want a plastic to be recyclable, it needs to be made of special ingredients, and/or made in a special way.  Not all plastic is, and so not all plastic is recyclable.

Similarly, for a plastic to be biodegradable, it needs to be made of special ingredients.  The most important thing about making a plastic biodegradable is that it has to be tasty to certain bacteria.  Bacteria are what "eat" biodegradable plastic and turn it into things that are safe for the environment.  Normal plastic like we talked about in the last paragraph usually isn't very tasty to bacteria.  To make biodegradable plastic, either the plastic itself needs to be made of special ingredients (again! plastic is complicated), or else it needs to have special ingredients added to it.  The first kind is better, but the second kind is popular because it lets companies use the ingredients they already have, but *also* make their plastic a little bit biodegradable.