system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes something "biodegradable"? What are the conditions necessary for something to biodegrade?
gpt
it needs to be edible to little tiny creatures.

Foods biodegrade because yeast and fungus and other little beasts find it and eat it.  they break it down and poop it out and if everything goes right give you really nice dirt with which to grow new foods that will also biodegrade.  This is what a compost pile hopes to achieve.

If we make a something out of a material that little creatures don't eat... it'll just stay in it's current form for a long long time.  A lot of plastics aren't biodegradable because no little yeast-bros ever evolved to eat it (you know, because it wasn't around to eat until recently)

it's really as simple as that.  if the material is able to be eaten by something it can return to the earth.  If the material can't be eaten by something it's stuck the way it is.