How did Shrinky Dinks work?

The file

Plastic sheets you colored and put in the oven, then watched curl, panic, and flatten into a tiny hard version of your drawing. It was the closest a '70s kid got to chemistry, and it smelled like it.

Two mothers invented it as a Cub Scout project and it became a permanent craft-aisle fixture. The licensed sets, Star Wars and Smurfs and the rest, are the ones people remember.

Sources: Wikipedia — Shrinky Dinks