Mirage is an optical phenomenon very common in sunny days. It’s caused by the redirection of the reflected light rays form the object. Is a real physical phenomenon and not just an optical illusion. The sun light in the direction of the road gets refracted do to the temperature gradient (continuous change) of the layers of air. This refraction causes the redirection of the sun rays and finally it gets reflected (total reflection) from the layers of air near the surface of the road.
On a hot day,the sun will come down, it will heat the surface of the road and make the air close to the road very hot in comparison to the air above it. That means that light rays are bent away from the road, and so, when you look down at the road, the rays are actually bending away from the road and back up into the sky, and you’re seeing a patch of the sky in the road. Water also looks like that because water is reflecting the sky, making the sky appear in the road. So the mirage looks exactly like water.