Kids these days have so many electronic toys it's possible to grow up without ever discovering the best toy in the world, the tin can telephone. Moms and Dads, take note! Lots of science-types agree that kids will remember a toy you build with them years after they have forgotten the latest plastic gizmo.
So, how do you make a tin can telephone? Take two metal cans and cut off one end on each. Be sure not to leave any sharp edges! If there are some, you can cover the rim with electrical tape. Make a small hole in the bottom of each can. Now take a long piece of string. Thread the string through the hole in each can, and tie a knot in each end so it can't pull back through the hole. Now get two kids to hold the cans so that the string is taut. As one speaks into a can, the other listens in the other can. You'll be surprised at how well this works.
Talking causes little vibrations in the air in front of your mouth, as you can tell by speaking into your hand. When you send those vibrations into the can, they cause the back end to vibrate. This vibration is passed along the string and into the other can. When that can vibrates, it creates vibrations in the air once again-and you can hear the other person's voice!
Tin can telephones are fun to make, and they're a great way to learn about how sound works.