Ron Henrie

Here's a cool scientific nuance: the freezing point and the melting point are the same temperature—0°C. This is known as a phase equilibrium. At exactly 0°C and 1 atm pressure, water and ice can coexist. If you add heat energy to the mixture, it will melt the ice without changing the temperature. If you remove heat energy, it will freeze the water, again without changing the temperature. The energy required for this phase change is called the latent heat of fusion. This is why a slurry of ice and water is such a stable thermal reference; it will stubbornly remain at 0°C until all of one phase is converted to the other. This principle is used in calibrating thermometers.