Water can boil and freeze at the same time at a specific temperature and pressure.

The triple point of water happens at 0.01 degrees Celsius and a very low air pressure of 611.657 pascals. This pressure is about 160 times lower than the air pressure at sea level. At this exact point, water is in a state of balance. Ice melts into water at the same speed that water freezes into ice. At the same time, liquid water turns into steam as fast as steam turns back into liquid.Scottish physicist James Thomson first discovered this concept in 1873. He found that every substance has a unique point where all three states can exist together. To see this in a lab, scientists use a sealed glass container called a triple point cell. They remove all the air and fill it with pure water to reach the correct pressure.For over 60 years, this specific point was used to define the Kelvin temperature scale. The definition only changed in 2019. In this state, adding heat does not make the water hotter. Instead, the extra energy just makes more ice melt or more liquid boil. The temperature stays exactly at 0.01 degrees Celsius until one of the states of matter completely disappears.