To keep spinning, yo-yos rely on reducing friction. To do this, they feature an axle wrapped in a machined concave bearing. If a thrower wants to bring the yo-yo back up, he or she must jump the ...

Such yo-yos lose only a trickle of rotational energy to the slight friction between the hard balls in the bearing and their races. Today’s yo-yo is a far cry from the first one to have a ball ...