Ice slips away from under a skate blade like a bar of soap in wet hands. For a hundred years, scientists argued about where this slipperiness comes from. Some said there is already a thin layer of water on top: it acts as a lubricant. Others countered that water only appears when you start moving, due to friction. In a new study, physicists combined both approaches, and found out who was right.

First, they built a computer model of ice sliding against glass at the level of individual atoms. The model showed that if you consider only nanoscale effects, friction comes out too high. So the researchers added what happens during real sliding, frictional heating, to the calculation. It turned out that even at a speed of just 0.1 meters per second, the temperature at the contact point jumps to nearly zero degrees Celsius — the melting point of ice. The ice begins to melt right under the skate blade or runner, and the water acts as a lubricant.

The calculations matched experimental data for a wide range of speeds — from slow sliding to fast skating. So the main secret of ice is not a pre-existing film, but the heat generated by motion. Ice lubricates itself when you start moving. It is as if the road melts under your feet just enough to let you keep gliding.