 Another consequence of the strong hydrogen bond is that the vapor pressure of water is extremely low. Now the vapor pressure can be thought of as a measure of how hard or easy it is to evaporate a liquid. When you have a low vapor pressure it's very difficult to evaporate that liquid. When you have a high vapor pressure it's very easy. Now the vapor pressure of water is incredibly low. Heating up water actually increases its vapor pressure and the boiling point of any liquid is defined as the temperature where the vapor pressure matches the atmospheric pressure. This means that we can not only boil water by increasing its temperature but we can boil water by decreasing the atmospheric pressure or the pressure that that water is in. So while we all know that water boils at 100 degrees Celsius this is only true for the reference atmospheric pressure which is at sea level. So as you go to lower pressures which you can achieve by going to higher altitudes for instance the temperature of the boiling point reduces. So for example water boils at 68 degrees on Mount Everest and we can demonstrate this phenomenon by reducing the pressure in this vessel which contains water at room temperature.