Ask an adult to punch a small hole in the bottom of each of the cans (from the bottom, inward), using the hammer and nail. Be very careful during this experiment, as the opening of the can and the holes can be sharp and dangerous.
Thread the string through the hole in the bottom of one of the cans and pull it through the top of the can. Do not reach your hand inside the can, which is sharp and can cut you. Instead, push the string through from the bottom.
Tie a large knot in the string on the opening-side of the can. The knot should be large enough so that the string won't fall back through the hole.
Repeat steps 2-3 with the other can, using the other end of the string. When you're finished, both cans should be facing opening-ends away from each other, with the rope connecting them.
Find a labmate (the adult who helped you earlier counts too!) to have your first tin-can telephone call with. Have your labmate put their ear near the can, but not inside of it. With the string loose between the two cans, try whispering a secret message into your can telephone. Can your labmate hear anything?
Now try holding the string tight between the two cans while whispering your message. Did that make a difference? Why do you think that is?
Be sure to take turns talking and listening with your new can telephone. Can you have a whole conversation?