Goop (not Gwyneth's one)

Mix three dessertspoons of cornflour with about one and a half of water. (just keep adding either until you have a solidish mixture.)

Goop! Mold into a ball and keep your hands moving - it keeps its shape. Stop and it turns wierdly liquidy. SO cool.

And even cooler, so far it has been easy to clean up.  (off the floor and countertop, that is. I have no idea if it will come off clothes.) Oh, and we added food colouring too.