Hold up a cup. Explain that it appears empty. We will conduct an experiment to determine if it really is. Turn it upside down so the tube is in a dish of water. Use a candle to heat the bottle - observe.
Bubbles blow into the water through the tube.
Add water to the bottle. The tube should reach the water in the bottle and still extend 2 inches beyond the stopper. Heat the flask. The water will return to its previous position. List conditions that could cause air to expand and contract.