Tin-can telephones do work, actually. The concept is pretty basic:
When a person places her mouth inside the opening of a can and talks, the sound from her voice vibrates against the bottom of the can.
These vibrations travel down the string and make the bottom of the other person’s can vibrate, too, transmitting the speaker’s voice into the listener’s ear.
What works even better than a tin can is a paper cup. It has less resistance, so the sound vibrates even better. But it’s imperative that the string be pulled tightly all the way across, or it won’t vibrate well enough to transmit sound.
This concept is actually very similar to the way a telephone works. While a telephone uses electricity to transmit the sound vibrations, the principle is the same.
It’s just as we always thought: play is educational.