The Mpemba effect is surprising: hot water sometimes freezes faster than cold. Scientists built a simple model: a bead in a thick, honey-like medium, on a landscape with two dips and a wall at the edge. The bead moves viscously, unable to hop over the central bump. The hot bead, having more energy, darts around wider, hits the wall more often, and after bouncing off, rolls into the target dip, settling down. The cold one moves sluggishly, lagging behind. The wall is key: without it, the effect vanishes in an infinite space. The dips can be any shape — symmetry isn't required.
This simple mechanism helps explain puzzles across sciences — from chemistry to materials science.
The effect was discovered by a Tanzanian schoolboy who noticed that hot ice cream mixture hardens faster than cold.