The Falling Sand Toy (called 'HELL OF SAND') is an ideal 21st Century replacement for the clacking balls and sand trays of the 1980s office. You can direct sand, water, salt, oil and fire around, with gravity and platforms you can draw. It has no point whatsoever but it's pretty cool to play with. Play with it now.

(Update: There's a whole Wikipedia article about it which goes into all the details of how it works and what all the parts do.)