Here's what most people assume when they watch a wave: the water is moving forward. 

A wave rolls across the ocean toward the beach, and it looks like water is travelling. But the duck in your bottle just showed you that this isn't what's happening.

A wave is not a movement of matter. It is a transfer of energy through a medium.

When you tip the bottle, you give energy to the fluid at one end. That energy disturbs the particles — pushing them slightly, which pushes the particles next to them, which push the next ones, all the way to the other end of the bottle. The energy travels through the medium. Each particle moves a little — up and down, in a small circle — and then returns to where it started.

That's why the duck bobs vertically and ends up back where it started. It's being moved by passing energy, not by travelling water.

You can see the same principle at work in [The Wave Factory], where the wave structure makes the energy transfer visible in a completely different way. And in [The Whisper Wire], sound does exactly the same thing through a string — energy passes through the medium, particle by particle, while the string itself goes nowhere.

Curiosity spark: If particles just move in circles and return to where they started, why do ocean waves eventually crash on a beach? The wave doesn't change — but something about the beach does. What do you think it is?

Find out in The Crazy Scientist Lab.