Video narration
Why does a distant road look wet in the sun, then dry as you get closer? On hot days, the road heats air just above it more than air higher up. The lowest air is hotter; the air above is cooler. Sky light passes through these layers. Their temperature difference bends its path, curving it toward your eyes from near the road. Your eyes trace light back in a straight line, so the sky seems to sit on the road. From far away, it looks like a shimmering puddle; as you approach, the angle changes and the image seems to slip away. That patch isn't water, and the road isn't reflecting the sky. The watery image is actually sky light, bent through hot air-a mirage, an apparent image created when light bends. So the road looks wet because bent sky light reaches your eyes from above it.