As a liquid is heated, its temperature increases and the molecules of the liquid gain energy and their kinetic energy increases. As the kinetic energy increases, the molecular motion increases and the molecules of the liquid overcome the force of attraction between them.On continuous heating, a particular temperature is reached where the molecules of the liquid leave the surface in the form of vapour to produce a pressure above the liquid equal to the atmospheric pressure and the liquid starts boiling. At this stage temperature remains stationary even on heating further. This stationary temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure is called the boiling point of that liquid. At this temperature the bubbles begin to form and rise in the liquid.
This video explains how to determine the boiling point of water.