A tin can telephone is not just a classic children's toy, it is also a crafty way to learn about how sound waves travel. Make a set of tin can telephones with your children so that they can learn how the sound waves from the words they speak cause the string to vibrate, carrying the waves (and the voice) to the person on the other end. Experiment with different lengths of string to see how the length affects the clarity of the conversation.
Wash both tin cans with warm soapy water and a sponge. Make sure the cans you choose do not have sharp jagged edges that may cut the child's lips or ears. Hammer down and sand any jagged edges until they are smooth.
Flip the cans upside down and place them on your work surface. Punch a small hole in the centre of the bottom of each can with a small nail and hammer.
Cut a length of kite string 1.8 metres or longer. Thread one end of the string through the hole in the bottom of one of the cans from the outside to the inside. Tie knots in that end of the string, one on top of the other, until the resulting knot is too big to slip through the hole. Repeat with the other end of the string and remaining tin can.
Give someone one tin can while you hold the other. Walk away from one another until the string is pulled taut. One person talks into his can while the other holds her can up to her ear. Take turns talking and listening.
Educate your children on the physics of waves. Ask them to observe the vibration in the string, and explain that these are sound waves made physical. Explain that the chamber of the can focuses and amplifies the sound.