system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What is plastic made of and why do we make so much of it?
gpt
Plastic is made of dead dinosaur food: let me explain!

Plastics are a general category of different materials: Plastic bags are made of polyethylene, bullet proof glass is made with polycarbonate, teflon is good for slippery chemical resistant coatings, polyester is often used in clothes, PET is used in water bottles, PVC is used for pipes. What all these materials have in common is they are generally big long tangled chains of carbon and hydrogen atoms (with other elements mixed in). 

Oil is often used as the starting material (feedstock) of plastics because it also is made of long chains of carbon and hydrogen. It can be reacted in different ways to rearrange the chains to form different plastics.

Oil is made of decomposed biomass from a long time ago that has been transformed by time and pressure below the earth's surface.  That biomass is mostly plants and the like from long ago, and could've been eaten by dinosaurs.

Plastic as a category makes a great material in many applications because it's cheap, light, and relatively resilient. As an example, think of disposable medical syringes. It's easy to make syringes by molding molten plastic. It's cheap, so we can just throw away the syringes instead of cleaning and reusing them. It's light: imagine the weight of glass syringes and the cost in shipping them. It's durable: imagine dropping a glass syringe versus a plastic one.