Well the myth was that if all the safety features of a water heater were disabled the water heater would go 500 feet in the air. They used a smaller one and it just released a large amount of superheated smoke. Then they used a bigger one and it went up in the air and came down 40 seconds later. Then they put a roff over it and the roof changed to 1000000 small pieces of building (don't worry they built it themselves)
Mythbusters is on SBS at 7:30
