ELI5 - How does a Tin-Can phone work?

A tin can phone is a simple way to send sound using nothing but two cans and a string. It feels like magic, but it is really just sound and movement working together.

When you talk, your voice makes the air move. Those tiny air movements are called sound waves. Normally, they travel through the air to someone’s ears. In a tin can phone, the sound takes a different path. You speak into one can. The bottom of the can is thin and flat. When your voice hits it, the bottom starts to shake. It shakes in the same pattern as your voice. Loud sounds make big shakes. Quiet sounds make small shakes.

A string is tied to the bottom of that can. When the can shakes, it pulls on the string. This sends the shaking along the string, like a tiny ripple moving through it. The string carries the movement, not the sound itself. This is important. The string must be pulled tight. If it is loose, the shaking gets lost. A tight string lets the movement travel cleanly from one end to the other. That is why tin can phones work best when the string is straight and stretched.

When the shaking reaches the other can, it makes the bottom of that can shake too. That shaking pushes the air inside the second can. The air then moves toward the listener’s ear. Your brain hears those air movements as sound. To the listener, it sounds like your voice came through the can. Nothing is being amplified. Nothing is using electricity. The sound is not traveling through the air between the cans. It is moving as vibrations through solid objects. First the can. Then the string. Then the other can.

This works because sound can travel through solids, liquids, and gases. In fact, sound often travels faster through solids than through air. The string is acting like a simple bridge that guides the vibrations where they need to go. Tin can phones stop working if the string touches something else. If it rubs on a table or a wall, the vibrations leak away. They also stop working if the string is too long or too stretchy. Strong, thin string works best.

This toy shows a big idea in a tiny way. Sound is just movement. If you can guide that movement, you can send a message without wires, batteries, or screens. A basic walkie talkie.