POP CAN: A small amount of water is placed in a pop can. The can is placed on a hot plate and heated until steam is coming out the top. The can is grasped off the hot plate and turned upside-down into a beaker of cold water. The condensed steam reduces the pressure & the can collapses.
55-GALLON DRUM: The same process can be used to collapse a 55-gallon drum. Water in the drum is heated with a large propane burner. The drum is sealed and the flame turned off. The drum is then cooled with spray bottles. When the steam condenses the drum dramatically collapses. Available on video tape. (Live version available depending upon work load.)