Has anybody noticed that polystyrene (styrofoam) tends to squeak? It's not as annoying when the foam squeaks, but solid polystyrene squeaks almost like claws on chalkboard. Try it for yourself. Take some polystyrene (transparent chocolate boxes or CD covers are an easy source) and rub it across the table.
I wonder what properties of polystyrene make it squeak easily, and what other materials squeak as well?