The goal is to get an empty pop can to implode (crush in on itself). To do so, take a pop can and empty the contents. Place a tablespoon of water in the bottom of the can and place the can directly on a stove burner. Let the can sit on the burner until you start to see steam coming out the top. Let it sit for another minute. Now take a pair of tongs and quickly, but carefully, use the tongs to pick up the can, tip it upside down, and submerge the can open side down into a large bowl of ice water. The can will immediately implode making a loud noise in the process!
The video above is our attempt at this experiment. It worked great and I assure you I didn't crush the can with the tongs! It happened so quickly that I didn't get the phone over in time to capture the video of the crush itself. I'll have to do better next time.
So what's happening? The can and air inside the can is very hot. By submerging the can in the ice cold water open side down, the air inside the can can't escape. Due to the drastic change in temperature from hot to cold, however, the air inside changes density very quickly and the can caves in on the sides. Cold air takes up less space than warm air.
If you still don't believe my video then I challenge you to try it out for yourself. Very cool!