By Harold McGee
If the temperature continues to rise and the molecules move with a kinetic energy high enough that they can break away from each other’s influence completely and move freely into the air, the substance become a different kind of fluid, a gas. The most familiar transition from the liquid phase to the gas phase is boiling, in which we transform liquid water into bubbles of water vapor, or steam. Less obvious to the eye, because it’s so gradual, is the evaporation of water at temperatures below the boiling point. The molecules in a liquid move with a wide range of kinetic energies, and a small portion of the molecules in room-temperature water are moving fast enough to escape from the surface and move into the air.