Don't try this at home - you
could be seriously killed !
In fact, I in no way endorse anyone else trying this stunt ever.
The physics behind what you see in Exploding Watermelons Clip 1 and Clip 2 is actually very simple. What I do in both clips is the following:
1. Before starting I cut a hole in the top of the watermelon big enough to slide a 20oz soda bottle through, and then I reached in and dug out a cavity inside the watermelon big enough to hold the bottle even when the top part of the watermelon is placed back over the hole - kind of like carving a pumpkin except no holes are made other than the one at the top.
2. I pour enough water into the watermelon so that when I place the 20oz soda bottle inside it is completely full (i.e. overflows a bit). I practice putting the "lid" of the melon back on in such a way that it keeps the bottle inside (the bottle wants to float and pop out of the hole). I have found you need to use a plastic fork or a stick or something to keep the lid from popping off since the bottle pushes up on it quite hard. Putting the bottle in the melon and covering the hole has to happen quickly when it is done with liquid nitrogen in the bottle, so I practiced a few times.
3. I remove the soda bottle from the watermelon, add some more water to the melon for good measure, and put the top of the watermelon nearby so I can quickly cover the hole in the top of the melon after the next step.
4. I pour a few ounces of liquid nitrogen into the soda bottle and screw the lid of the bottle on very tight. The liquid nitrogen starts to immediately boil inside the bottle and the pressure builds up quickly, so the next steps have to happen very quickly (a few seconds).
5. I shove the bottle into the watermelon and cover the hole with the top of the melon.
6. Run for cover.
The pressure builds up inside the soda bottle until it ruptures and explodes. The water surrounding the soda bottle inside the watermelon serves as a supply of heat to make sure the liquid nitrogen boils rapidly, and also transfers the shockwave created by the explosion from the soda bottle to the walls of the watermelon, which blows it to tiny bits. See the video clips.