Hot Air Balloon Experiment to demonstrate how hot air rises
Glass beverage bottle
Hot water ( from kettle)
Saucepan / bowl
Put the lip on the balloon over the glass bottle and let the balloon hang limply over the outside of the bottle. Boil the kettle then put the bottle in a pan of very warm /hot water, taking care when pouring. Then watch the balloon start to rise.
Explain to the kids-
hot water will warm the air inside the bottle causing air to expand and move the balloon up inflating before your eyes.
For hot air balloons to rise there they must be filled with air lighter than the air around it and warmer air is lighter than cool air. so when the air inside the hot air balloon is heated, air molecules speed up and spread out. This makes hot air less dense, therefore lighter.