A video tutorial on how to build a contact trigger for your flash. This will let you do high-speed flash photography of interesting things like balloons bursting, light bulbs exploding, etc.

A projectile (like a BB from a BB gun) goes through the two layers of foil, mashing them together and closing the circuit, which fires the flash. The BB continues on to go through a balloon or other such thing. The camera is left on a long shutter speed (say 1 to 5 seconds) and the flash exposes the action as it happens