A mirage is a kind of optical illusion that occurs in hot, still weather. A pool of water suddenly appears in the distance. But on approaching, it seems to evaporate and there’s actually no water at all. You may see these mirages on roads in summer. They also occur in deserts, sometimes complete with trees, and may trick a thirsty traveler into thinking that an oasis lies ahead.
What you really see is simply a reflection, usually of the sky. The reason is that a layer of hot air near the ground acts like a mirror.