Boiling occurs when bubbles form within a liquid, marking a change from a substance’s liquid or solid phase into a gas. The normal boiling point is the temperature at which the liquid’s vapour pressure equals the standard sea level atmospheric pressure (760 mm [29.92 inches] of mercury). At sea level, atmospheric pressure is high, and water boils at 100 °C (212 °F); at higher altitudes it is lower, so water boils at a lower temperature.