We called it Magic Mud in ECE. Just throw a bunch of cornstarch in a large basin (probably a couple boxes, maybe 3). Then add some water and mix a bit at a time until it's goopy. It should have the consistency that allows you to have a little difficulty picking it up (seemingly solid) then will ooze between your fingers like liquid. Very cool activity. Can add some colouring to it for fun. Don't try to keep it for future use though....I tried....it's gross. It's an awesome activity!