Cilia are tiny hairs on cell surfaces. They beat like oars, moving fluid. In the lungs, they push dust and microbes out. But their lengths vary: in humans, they are thousandths of a millimeter; in comb jellies, several millimeters. How do such different cilia work together?

Scientists built a machine that jerks fluid sharply, as if a cilium had struck. They filmed it with a high-speed camera. It turned out that after a sharp push, the fluid keeps moving by inertia for a long time. If the pushes come in a wave, particles slide along it like surfers — from one cilium to the next. This is highly efficient transport.

Such behavior is impossible in a fluid as thick as honey — everything stops right after the push. But in water, like a river, inertia aids motion. This discovery could help create artificial cilia for water purification or drug delivery.