Auxetic materials have negative Poisson ratio, which means when you stretch 'em, unlike a rubber band, they get thicker. Sketches of structures that do that are cool to look at. Like rug patterns or chunks of Escher drawings or cool internet wallpaper imagery. Here's a gif (thanks to Joseph N. Grima of the University of Malta) that shows a way it works, if you're wondering and having trouble imagining:
Apparently there are YouTube videos of auxetic foam, but I didn't go that far down the rabbithole.
