Exhibit: Hot Air Balloon
Press the red button to heat the air inside the balloon. When the air is at about 90 degrees C, press the green button. The balloon rises into the air and comes down again when the air has cooled. Hot air rises, so if it is inside a balloon, it might be able to lift it up to the ceiling. The air inside the balloon is heated by a powerful blower. This blows cold air all the time to keep the balloon inflated, but when you press the red button, it blows hot air instead. The balloon is soon full of expanded hot air, which is much lighter than the cold air outside the balloon. As the balloon rises, the cold air cools down and contracts. The balloon gets heavier and descends, ready for the next trip.