Try this one. The maintenance guy at our local dairy, well the only dairy, this is a small island. He used to cut an opening about 2 to 3" sq into the side of an old 1 gallon metal oil can. Put the lid on and then purge a little Ammonia into the can. After which he would take a liquid dropper and drip one small drop of water through the opening. The can would collapse in on itself!