The freezing water in this exhibit depends on two factors. First, water cools when it evaporates. Second, water evaporates more rapidly when one pumps away the air and water vapor. Move the switch to VACUUM. The valve will open and allow the pump to start pumping air out of the chamber. Watch the vacuum guage. Add a little water with the valve marked water. In about 30 seconds the water will begin to freeze. Move the switch to AIR and wait for the air to re-enter the chamber. Watch the vacuum guage. Now you can open the door & remove the ice (Note: water will not come into the chamber unless there is a vacuum inside).