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. Likewise, if you're not rotating, you can't rotate unless a torque starts you rotating. There's just one more important detail.