Nature abhors a vacuum, or so the saying goes, and you can use that to your advantage when it comes to building a giant table-sized cannon that can obliterate a watermelon without the need for dangerous explosives or compressed air tanks.
Submarines diving to the deepest parts of the ocean need to be built incredibly strong to survive the extreme pressure of all that water. The air in the earth’s atmosphere does the same thing—you just don’t really feel the pressure because you’ve been living your whole life exposed to it. But break the seal on the end of a ten-foot plastic tube with a vacuum inside, and that atmospheric pressure will send air rushing in so fast it can send a projectile hurdling out the other end at close to the speed of sound. [YouTube]