A mirage occurs when light changes direction as it moves between batches of air having different indices of refraction. The different indices of refraction occur because the air has different densities at different temperatures. Two images are seen; One from a direct path from the object to you, and the second arriving by rays originally heading toward Earth but refracted to your eye. On a hot day, the Sun makes the surface of blacktop hot, so the air is hot directly above it, becoming cooler as one moves higher into the sky. The “water” we see far in front of us is an image of the bluesky. Adding to the effect is the fact that the image shimmers as the air changes in temperature, giving the appearance of moving water.