The kamifusen is a traditional Japanese toy made of colorful paper. It resembles a beach ball, but unlike that toy, the kamifusen has an open hole at one end. Given that hole, one might expect the toy to deflate when struck, but the opposite is true – a deflated kamifusen inflates itself when bounced. The key to this counter-intuitive behavior comes from a combination of fluid dynamics and solid mechanics.
When the kamifusen bounces off a player’s hand, it is compressed, which increases pressure inside the toy and forces some air out. Elastic waves rebound through the ball’s paper walls, much like seismic waves traveling outward from an earthquake. Those waves re-expand the toy’s walls, dropping the interior pressure and pulling air in from the outside. Although the pressure spike from impact is larger, its duration is short compared to the low pressure generated by the subsequent elastic waves. As a result, more air flows into the toy than is knocked out, and so the kamifusen inflates. For more, check out this explanation at Physics Today. (Image and research credit: I. Fukumori, source; submitted by E. van Andel)