This was the question I asked my son when we started our crafty experiment. He didn't have an answer for me then, but by the time we put our mobile to use, he definitely did!
What You Need
string (we used embroidery thread)
an circular object to trace, roughly 4 inches in diameter
How to Make It
Decide how many circular wheels your mobile will have. One works just fine but more is fun too. Layer pieces of construction paper, and trace a circle on the top piece. Cut through all the layers to create your circles.
Fold each in half, then in half again, and in half one more time. Make sure to crease the folds well.
Unfold the circles and mark approximately 1/2-inch from the edge on each fold.
Cut up to each of these marks and fold back the paper on one side of each slit.
Use a push pin to poke a hole in the very center of each circle. Wiggle it to make the hole big enough to fit the string through.
Tie one end of the string to the paperclip. Thread the other end through the hole in the center of the circle, making sure the folded corners face down.
What to Do With It
Hold the mobile steady directly over the bulb of a table lamp. Observe what happens to the mobile's circles - they slowly begin to turn!
Note: Some high efficiency light bulbs give off less heat. Edison and incandescent work well. Try different lamps, different distances away from the bulb, etc. Just make sure to warn kids not to touch a light bulb that may have been illuminated for awhile.
How it Works
The lamp doesn't just make light, it makes heat too. As it warms the air above it, that air rises while cooler air becomes heavy and sinks. As the warm air cools, it eventually sinks. The air is constantly moving as it warms and cools. The warm air above the lamp begins to rise and this air movement makes the circles on your mobile spin!
This great idea came from the book Super Science With Your Kids. It has LOADS of great activities and ideas; check it out!