To boil something means to heat it to the point where it reaches its boiling point, which is the temperature at which the liquid turns into vapor—bubbles of the vapor rise to the surface and burst, releasing the vapor into the air. The boiling point of a liquid depends on its chemical composition and the surrounding pressure. For example, the boiling point of water at sea level is 100 degrees Celsius (212 degrees Fahrenheit), but it boils at a lower temperature at higher altitudes.