Building a cannon in your backyard is good clean fun (if you're responsible and not a jerk!), but maybe you don't have all the stuff you need to make it happen. Maybe you don't have a compressed air tank to generate the force necessary to fire a projectile, or perhaps you have some sort of ethical issue with explosive fuel. We're not ones to judge, but the good news is that even without those things, you can still create a watermelon-destroying cannon that's powered by something all around us: the Earth's atmosphere (via Sploid).
This type of cannon is called a bore vacuum cannon, and YouTuber NightHawkInLight explains just how it works and how to make your own. It works by creating a vacuum in the cannon's chamber, and breaking the seal on one end is "like blowing the hatch on a submarine while it's still underwater," as the video puts it. This means that designing one of these cannons is very simple. NightHawkInLight made his with a 10-foot PVC pipe, made holes to attach vacuum lines so he could later suck the air out of the chamber, and created a wooden base. Then he made "burst discs," which in this case are layers of aluminum foil secured by duct tape, on each end. All it takes to fire the cannon at this point is taking the air out of it and breaking one of the ends, which NightHawkInLight achieves with a simple mallet.