It was this tube of plasticky gunk, I think it was literally plastic, and you would squeeze out a little bit onto a little plastic tube (like a stirring straw) and then inflate it like a balloon. The rather awful-smelling chemicals in it would quickly evaporate, leaving a hard ball of stuff.

Kind of pointless, and undoubtedly toxic; it smelled like dry erase markers.

The plastic gunk was multi-colored, sort of like trident toothpaste, and the end result looked a lot like that photograph.