Desktop Shooting Gallery

This is one I was considering building just for myself. A series of clear tubes (let's say 5) is set up with ping pong balls lightly stopping the top. In the tubes is butane/air mix, and two electrodes. Under each tube is a photodiode/transistor/resistor. When it is hit by the laser pointer the sudden change in light level is registered and a spark is sent through the according tube. The butane ignites and BANG the ping pong ball flies into the air. This, of course, is a totally useless waste of time, but it sure beats those boring desk toys doesn't it? The tubes would probably have to be manually filled back up with butane or dimethyl ether or propane from some sort of a bottle or can. Some of those ozone friendly dust off cans work pretty well in spud guns, and this is basically a smaller version.