You know that liquid nitrogen turns into a gas at room temperature. Place some in a two-liter bottle for a physics demonstration of the ideal gas law. Beware, however; this is a dangerous demonstration! Not to mention that you may not have the time to fill a trash can full of water for each period that you teach in a day. If this is the case, you can find a video clip online of the same or similar demonstrations being done and simply include it in your lecture on the behavior of gases.