Physics
The Triple Point of Water
Quick fact
The triple point of water occurs at exactly 273.16 K (0.01 °C) and a pressure of 611.657 pascals (about 0.006 atmospheres). At this specific condition, all three phases coexist in a stable equilibrium, and it is the defining reference for the Kelvin temperature scale.
Why this is interesting
You've seen ice melt and water boil, but there's a single temperature and pressure where ice, liquid water, and steam all exist together in one jar. How can all three forms of the same substance be stable at once?