Carles Falcó

A crowd in a city square behaves differently depending on whether people hold hands. If each person only lightly touches a neighbor's elbow, people easily spread out in all directions — the crowd expands as a smooth carpet. But if they interlock fingers, movement slows down and the crowd breaks into clumps: dense clusters form here and there, with empty corridors between them.

A colony of cells resembles a squad blazing a trail through unknown terrain. In living tissue, this role is often played by chemotaxis — movement along a gradient of a chemical that the cells themselves secrete. Earlier models described homogeneous groups, but in reality, different cell types interact.