Oobleck is a milky-white, shiny substance known as a non-Newtonian fluid. It flows like thick paint when you pour it, but mash your hand onto its surface and it forms a hard skin. Squeeze some in your palm and it will form a tough glob. But the second you release it, oobleck trickles down over your fingers in a slurry. It's gross, it's fun, and any kid will be caught up in its magical ability to switch back and forth between a solid and a liquid.
Oobleck is actually a pretty simple mixture of cornstarch and water. Its common name (which I later learned is not what all kids call it) comes from a Dr. Seuss story, Bartholomew and the Oobleck, where a young boy's wish for something other than rain or snow to fall from the sky is granted. The characters in Seuss' book soon need to be saved from this sticky new form of precipitation, but real-life oobleck is far more benign, and scientifically interesting.