I've done this before. The can crushes because the air inside the can is rapidly cooled (and thus compressed) when it hits the water. It's like putting inflated balloons in liquid nitrogen, putting them in a cooler and dumping it out at a party. The air inside the balloons is compressed so much that the balloons look like they're deflated.

Looks fun to do. Might try this out. Probably can't, my mom always recycles them. I like getting money. :-)

while this is fun the first couple times... it gets to be a real pain after ur frat has a large party....