Here's how it works:

* Hot air is less dense than cold air. This means light travels faster through hot air.

* On a hot day, the air near the ground is much hotter than the air higher up. This creates a sharp change in air density.

* Light rays passing from cooler air to hotter air are refracted (bent) upwards.

* This bending can create an image of the sky that appears as a reflection on the hot ground. This creates the illusion of a pool of water.

Other examples of abrupt refraction include:

* The "disappearing" spoon in a glass of water. The spoon appears to be broken at the surface of the water because light bends as it passes from the water to the air.

* A prism separating white light into a rainbow. The different wavelengths of light are refracted at different angles, creating the rainbow effect.

Key takeaway: Abrupt refraction occurs when light passes from a medium with one density to a medium with a significantly different density, causing a sudden change in the direction of the light ray.