People find pogo sticks a source of entrainment, other find it as a hardcore sport. The pogo stick may never upend the wheel as a means of locomotion. But as inventions go, they share something: Once built, there wasn’t a whole lot anyone could seem to do to improve the basic design. In the more than eight decades since a Russian immigrant named George B. Hansburg introduced the pogo stick to America, the device had scarcely changed: a homely stilt with foot pegs and a steel coil spring that bopped riders a few inches off the ground. And bopped. And bopped. And bopped. Some kids fell off so many times they gave up, tossing the pogo next to the dinged hula hoops and unicycle deep in the garage. Others just outgrew it, gaining enough weight as teenagers to snap the stick or snuff the spring.