Vapour pressure of water

Vapour pressure is like a big air balloon that is filled with water. Water wants to escape from the balloon and turn into a gas called "vapour". The pressure of the water inside the balloon gets bigger and bigger as more water is added and it gets harder and harder for that water to turn into vapour. The bigger the pressure inside the balloon, the harder it is for the water to escape as vapour. Vapour pressure of water lets us measure how much water is inside the balloon and how hard it is to turn it into vapour.