In sunny weather, I always see a piece of water on the road ahead. What happened?

This phenomenon is caused by total reflection when light propagates in air with uneven density distribution.

In the air near the hot road surface, the rising hot airflow is mixed with transpiration water vapor, which is smaller than the refractive index of the upper air. Light emitted from distant objects to the road surface may be totally reflected. Seen from a distance, the road looks particularly smooth, just like a pool of water. This phenomenon disappears with the change of light angle after approaching, which is the same principle as what we usually call a mirage.