The fact that the ice is slippery is a matter of course for us. After all, most of us must have ice-skated, skied, or at least slipped on the icy sidewalk. So it may come as a surprise to find that physicists have been working on ice for more than 160 years, and we do not yet have a perfect and clear answer as to why this is the case. What do we know now and why is the ice actually slippery?
The coefficient of friction between ice and another body is too high for the body to slide on the ice. In 1850, physicist James Thompson predicted that the slipperiness of the ice was due to a small layer of water that forms at the interface of the two bodies. This assumption was correct, because by applying pressure to the ice up to 209.9 MPa, the melting point of the ice drops from 0 °C to -22 °C, but later findings have shown that the pressure of the average person's skate on ice causes a small layer of water to form only from -0.1 °C.
It was not until 1939 that a pair of English physicists were able to discover that water heating caused by friction played a crucial role at low temperatures. This heating then results in the formation of that thin layer of water on which the body slides on the ice. Only later experiments determined that this thickness is only units up to tens of nanometers for temperatures below freezing.
Today, we know that pressure and friction are not the only phenomena that contribute to ice slip. It is even possible that pressure and friction have a significantly smaller effect on the formation of a thin layer of water than previously thought. However, new hypotheses based on recent experiments still need to be further examined. Anyway, it is interesting to see how such relatively common phenomena can occupy physicists for decades or hundreds of years, and that even in times of amazing technological progress, we cannot answer many of them with certainty.