Polymers are made up of many many molecules all strung together to form really long chains (and sometimes more complicated structures, too).
What makes polymers so fun is that how they act depends on what kinds of molecules they’re made up of and how they’re put together. The properties of anything made out of polymers really reflect what’s going on at the ultra-tiny (molecular) level. So, things that are made of polymers look, feel, and act depending on how their atoms and molecules are connected, as well as which ones we use to begin with! Some are rubbery, like a bouncy ball, some are sticky and gooey, and some are hard and tough, like a skateboard.