Here's why:

* Refraction is the bending of light as it passes from one medium to another. This happens because light travels at different speeds in different materials.

* Water and air have different densities, and therefore, light travels at different speeds in each. This difference in speed causes light to bend when it enters or exits either medium.

Examples of refraction in action:

* A straw in a glass of water appears bent at the surface of the water.

* The shimmering effect seen on a hot road surface is due to the refraction of light through layers of air at different temperatures.