Why does water boil at 100 degrees centigrade?

Answer 1

It is a natural property of a liquid to boil at a particular temperature--or actually in a temperature range, because low atmospheric pressure can make the boiling point a little lower, or high pressure can make it a little higher.

That being said, the Celsius (a.k.a. Centigrade) scale was set up with 100 degrees being the boiling point of water with the atmospheric pressure at sea level, and also with zero degrees at the freezing point. Calling those temperatures 100 and 0 was arbitrary.

Answer 2

Water boils at 100 degrees Celsius because this is the temperature at which its vapor pressure equals the atmospheric pressure. At this point, water molecules have enough thermal energy to overcome the intermolecular forces that hold them together in the liquid phase, allowing them to escape as vapor.