system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what is actually happening when the air above a hot surface shimmers?
gpt
The air densities are different. The air right above a hot surface is hotter than the air higher up. Hotter air is less dense than cooler air, so the hot air rises and it mixes with the cooler air. Since the air has different densities it doesn’t mix uniformly (at least not right at the surface) and so the light refracts differently through the differently dense air, and it creates that shimmery, wavy effect. 

You can do the same thing with hot and cold water. Pour cold water into hot (slowly) and you can watch the two temperatures of liquid mix together. Before they even out, you’ll see a similar shimmery effect that you can observe in the air.