Here's a breakdown:

* Vapor Pressure: Every liquid has a tendency to evaporate. The pressure exerted by the vapor above the liquid is called the vapor pressure. This pressure increases as the temperature rises because more molecules have enough energy to escape the liquid's surface.

* External Pressure: The pressure exerted on the liquid's surface by the surrounding environment (usually atmospheric pressure).

* Boiling Point: The boiling point is the temperature at which the vapor pressure of a liquid equals the external pressure acting on its surface. At this point, bubbles of vapor form within the liquid and rise to the surface, causing the liquid to boil.

Key points:

* The boiling point is not a fixed value for a given liquid. It depends on the external pressure.

* At higher altitudes, where atmospheric pressure is lower, liquids boil at lower temperatures. This is why water boils at a lower temperature on Mount Everest than at sea level.

* The normal boiling point is the boiling point at standard atmospheric pressure (1 atm).