Have you ever looked along a smooth road in hot weather and seen what appears to be a stretch of water where none exists? This is a mirage. It is caused by the air being heated in such a way that hot layers of air lie under cold layers. The cold air is thicker than the hot air and this causes light passing through the layers to be bent. This bending gives the illusion of water, or even sends a picture of the scene many miles away.
Mirages often occur in the desert. Lakes are seen where there is really nothing but sand. And sometimes images of towns and palm trees appear and taunt the thirsty traveler. The same thing happens at sea when ghostly ships are seen floating in the sky.