What you need:
Several identical black plastic bags-you can use the packet of garbage bags available at a local store
Scissors
Duct tape (black or clear is preferable)
A pump
A string
A hot, sunny day
What to do:
1. To make the balloon, cut off the tops (handles) of each of the bags. Now cut off the bottoms of all the bags except two. (You can use five to seven bags)
2. Now, on a flat surface, arrange the bags in the shape of a tube. The mouth of one bag should overlap at least an inch with the bottom of another. The two ends of the tube should be the bags whose bases are intact.
3. Tape all the bags together in the shape of the tube (make sure the tape overlaps both bags enough or they will fall apart). Check the bags for any tears and tape them up too.
4. Once the long balloon bag is ready, cut a small hole into one end of the bag to inflate it with the pump. Seal it with duct tape once the bag is completely filled with air.
5. Tie a long string around one end of the balloon.
6. Take the balloon outdoors on a hot, sunny day. Hold the string or tether it to the ground and wait
What happens:
It takes some time (anywhere between 10 and 30 minutes) but the bag slowly rises and floats like a balloon. If not tethered, it will float away in the wind!
Why?
The bag is black in colour for a reason. Black absorbs heat really well. The outside of the bag gets heated in the sun and this heats up the air inside the bag too. As the air heats, it expands and its molecules spread out. This space between the molecules makes the hot air less dense (and hence lighter) than the surrounding cooler air. So the balloon rises up and floats above the cooler air.
Picture Credit : Google
