What is the boiling point water?

Boiling is the formation of vapor bubbles inside a liquid. They are not air, but rather some of the liquid that has evaporated.

The molecules of the liquid have to have enough energy to break out of the liquid phase and become the vapor. That means the vapor has to push the liquid aside to make the bubble. This happens when the vapor can push back against the atmosphere pushing on the liquid.

The boiling point is the temperature where the vapor has enough push to overcome the air pressure. In other Words, the boiling point is the temperature where the vapor pressure and the atmospheric pressure are the same. The lower the atmospheric pressure, the lower the boiling point.