Answer:
When it is very hot and sunny, roads can become very hot. When light rays from the sun reach this air pocket just above the road, the speed of the photon increases slightly, causing its path to alter, or bend from an observer's point of view. This makes something that looks like a puddle of water appear on the road.
People sometimes label a mirage as an illusion or as a hallucination. But, a mirage is neither one of those. Illusions and hallucinations are products of the mind. But the physics of Earth's atmosphere causes a mirage.
Mirages really have nothing to do with water at all. It's really all about how light travels through the air.
Here is a labelled diagram:
