FLOATING ON AIR: Like a fish in water, balloons float in air because their weight exactly balances the weight of the material they displace. Air expands when it is heated, so a balloonful of hot air is lighter than the same volume of cold air. If the combined weight of the balloon, its passengers and ballast, and the hot air inside it equals the weight of cold air that would fill the same space, the balloon will float. Heat the air a little more, or throw out some ballast, and the balloon will rise. Allow the air inside the balloon to cool, and it will gradually sink to the ground.

Heat can make things fly! Air rises when it gets hot, and hot air is powerful enough to carry a large balloon into the air. Make and fly your own model hot-air balloon.

• Scissors
• Ring of stiff paper
• Thread or string
• Card, cut and marked as a box
• Pencil, ruler, brush and glue
• Colored tissue paper
• Card template

1. Using the card template, cut out eight tissue panels. If you like use two different colors of tissue paper.
2. Glue the panels of tissue paper together. Glue the opening at the base to the ring of paper.
3. Fold and glue the card to make a box. Make holes in the top corners.
4. Use the thread to attach the box to the ring of paper at the base of the balloon.
5. Now your balloon is ready. To make it fly, hold it over a hairdryer. Switch it on and hold the balloon until it is full of warm air. Release the balloon, and it soars into the sky.

A real hot-air balloon has a basket hanging

beneath it and a gas burner above the basket.

Flames from the burner heat the air in the balloon.

The hot air is lighter than the cooler air around it

and it carries the balloon upward. The very first

people to fly did so in a hot-air balloon and today

people still enjoy flying in this way.