Why does the road look like it's wet on hot days?

       Have you ever wondered why roads are shimmering during Summer?

It is called Mirage: it is caused by hot air near the road and less hot air above it creates a gradient in the refractive index of the air and so making a virtual image of the sky appear to be on or below the road. Air currents make this shimmer, similar to a reflection of the sky on water, hence causing the illusion of wetness.