So I'm going to put it in the bell jar there, and I'm going to take all the air out so that the pressure goes down and down and down, but the temperature is not changing, you go down in the liquid phase, stays liquid... stays liquid... stays liquid... stays liquid until you reach this point, and then you have coexistence between the gas and the liquid-- we call that vapor, in the case of water-- and that means it will start to boil. 