system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What even is plastic?
gpt
You've probably heard that everything is made up of atoms. Well, atoms come together in different ways to form unique molecules. Sugar is a molecule, for example. 

Plastics are very special molecules called polymers. These polymers are made up of monomers, which are like lego blocks that can be stacked up on top of each other to make a bigger and bigger molecule. What makes plastics really special is that these molecules are very big and stable, and it's very hard to pull the lego pieces apart and break them down again. That is why plastics last for so long in the environment.