To catch oldfield mice, Hopi Hoekstra needed a long tube and quick reflexes. The mice dig burrows in sandy fields and beaches of the southern USA. They build a round nest chamber, an entrance tunnel that connects it to the outside world, and an escape tunnel that extends in the opposite direction and ends just below the surface. If Hoekstra put a tube down an entrance and blew through it, a mouse would erupt from of its escape hatch in a shower of sand.* And because the burrows are so standardized, it was easy to predict where the mouse will emerge.

Back in the lab, Hoekstra’s team, including Jesse Weber and Brant Peterson, made a surprising discovery. The shape