Here is a clever bit that looks like it was done in a mall...The setup - a Lego Mindstorm NXT robot that responds to noise...The more you make the faster it moves.
The trick here is that the end wall of the tank is F/X glass, so when the 'bot gets to the end it shatters. Gotta love the expression on peoples faces!
Thanks to Philo for posting up this one...If the flash doesn't work for you try the original page here.... (I've move the embedded video to the full view...)