At some point in every parent's life this question is going to come up. You can either hem and hah and make something up, or you can tell them the truth. And the truth is that plastics "consist of building blocks called hydrocarbons, typically derived from petroleum or natural gas. These monomers (small molecules) are bonded into chains called polymers or plastic resins. Different combinations of monomers yield resins with special properties and characteristics." There ... that wasn't so hard now was it?
