A skateboarder launches straight into the air from the top of a ramp. Seeming to hang in place for just a moment, he turns in midair and directs himself back down the ramp. Skaters call this maneuver a frontside 180. Physicists call it impossible.
Well, they don’t really call it impossible. Just very, very sneaky

You see, one of the fundamentals of physics is a little something called the law of conservation of angular momentum. This is what it says: If you’re rotating, you’ll keep rotating unless a twisting force, or torque, acts to stop you. Likewise, if you’re not rotating, you can’t rotate unless a torque starts you rotating. Seems fair enough, right? There’s just one more important detail. If you’re in midair, the only force that can act on you is gravity. On earth, gravity can’t make you rotate; it can only make you fall. So the question is, how does a skater go from not rotating to rotating, without the help of an external force?

As it turns out, skaters rotate in midair using a trick borrowed from housecats. This simple experiment will show you how and why it works.