When a wave bounces off a surface and returns, it's called reflection.

Here's why:

* Reflection is a phenomenon where waves change direction when they encounter a boundary between two different media (like air and water, or water and a solid surface).

* The wave doesn't change its nature (like its frequency or wavelength), but it changes its direction of propagation.

Examples of reflection:

* Light reflecting off a mirror: This is the most common example.

* Sound waves bouncing off a wall: This is why you hear echoes.

* Water waves reflecting off a seawall: You can see this happening at beaches.

Let me know if you'd like to learn more about reflection!