Super fun yet super tiring: jumping rope! Not to mention pretty difficult to get down. It actually takes quite a bit of physics you might not have thought of in order to time that jump perfectly and keep going.
First and foremost, jumping. Gravity forces you to have to jump up and over the rope as it swings under, otherwise the rope would whip your legs, probably leaving some red marks. It pulls the jumper down in between jumps so the cycle can continue.
So what about the rope? How come it doesn't hit you in the back of the head when it's on the top of its rhythm? That's due to a centrifugal force, pushing outwards, caused by the circling of the jumper's wrists, which keeps the rope in a uniform circular motion. Generally, to get the timing of your jump to be spot on, you should jump when the rope is at eye level, on its way down. Just try to maintain a constant speed and practice makes perfect!