Here's why:

* Vapor Pressure: The pressure exerted by the vapor of a liquid in equilibrium with its liquid phase.

* Boiling Point: The temperature at which the vapor pressure of a liquid equals the surrounding atmospheric pressure.

At higher altitudes, the atmospheric pressure is lower. For a liquid to boil, its vapor pressure needs to match the surrounding pressure. Since the pressure is lower at higher altitudes, the liquid reaches its boiling point at a lower temperature.

Example: Water boils at 100°C (212°F) at sea level, but at higher altitudes, it boils at a lower temperature. This is why cooking takes longer at higher altitudes.