Why does water boil at 100 degrees?

Water boils at 100 degrees Celsius because, at that temperature, its vapor pressure equals the pressure of the surrounding atmosphere at sea level. The molecules of liquid are moving too energetically for the outside pressure to keep them in the liquid at boiling temperature.

Two forces hold liquids together: the pressure from outside and the forces of attraction from outside. The molecules of a liquid are constantly in motion, and some are always escaping from it into a gaseous state. At boiling temperatures, all of the molecules are trying to escape the liquid; each one that does uses some of the heat energy, so it takes time for a liquid to boil completely.