A thin stream of water flows down a cold inclined board and, before your eyes, turns into an ice ridge. At first, the water paves itself a straight icy path, as if reinforcing its own banks. Then the trickle starts to meander, and the ice quickly spreads sideways. Finally, the ridge stops growing outward and begins to thicken, smoothing out irregularities.

A thermal camera showed that the time water spends on different parts of the board follows a simple rule: more in the center, less at the edges. It's like a worn path: the more people walk over one spot, the deeper the rut. Only here, instead of feet, it's the heat of the water, and instead of soil, it's the growing ice.

The scientists derived a formula that predicts the height of the ice at any point. The formula says: the farther from the start of the trickle, the higher the ice, and it grows exponentially — slowly at first, then faster and faster. Tests in experiments showed excellent agreement. Such calculations will be useful for predicting how ice builds up on wires, roofs, or airplane wings.