Oobleck!

This is a cheap and easy kids' activity that is entertaining even for adults. Mix together corn starch and water - about 1.5 to 1 in proportion - add food coloring for effect - and start playing.

I haven't a clue how to explain the physics, but if you scoop up a bit of it, it will run through your fingers. But clench your fist around it quickly, and instead of squirting out of your hand, it becomes rock hard (well, hard like a soft rock made out of corn starch).

The name "Oobleck" comes from the Dr. Seuss story, "Bartholomew and the Oobleck." Anything associated with Dr. Seuss has to be fun - if you've never played with Oobleck, give it a try. It's sure to bring out the kid in you.