Water boils at 100 degrees Celsius under standard atmospheric pressure, which is the pressure at sea level. At this temperature, the molecules in water gain enough energy to transition from a liquid state to a gaseous state, forming water vapor. This boiling point can change with variations in pressure. For example, at higher altitudes where atmospheric pressure is lower, water will boil at temperatures less than 100 degrees Celsius. Conversely, in a pressure cooker, water can boil at temperatures above 100 degrees Celsius due to increased pressure.