Why do string telephones work?

You will need two tin cans and some string. Punch a small hole in the center bottom of each can, and run the string through each hole and tie a know on each end. Position the two cans so that the string between them it taught ... the voice will resonate inside the can and transmit that across the taught string and into the other can. One talks into the can and the listener puts the other can up to their ear.

If someone were to touch the string part way along, their finger would dampen the waves travelling along the string.