Goop, better known as Oobleck (named for a slime in Dr. Seuss's book Bartholomew and the Oobleck ) is a fun material to play with: At one moment it's a solid, and at the next it's a liquid it's unbelievably silly to play with, and I've witnessed adults get lost in the strange sensation of its texture. For my science friends out there, this is a dilatant material, which is one that changes its properties in reaction to external stimuli. We don't have the Dr. Seuss book (yet!), but I imagine it would be fun to read the book in conjunction with this activity.