system
Given the following post, respond as if you're commenting on Reddit:
human
When I "see heat waves" rising from a hot, dry street, what is it that I'm actually seeing?
gpt
The heat is changing the density of the air, because heat makes things expand.  This in turn changes the refractive index of the air, and hence the direction through which light rays pass through the air.  The ripple, reflection-type illusion is the light rays refracting differently through the air.