When you take a container with water and begin to heat it, after a while, you will notice that bubbles begin to form. If you wait a bit longer, you will easily (it's so clear) start noticing smudgy heat waves in the water (which I believe is due to change in refractive index of the water). These waves tend to occur at discrete intervals and it takes quite a time for the first wave to be noticed.
What exactly are these waves and how are they created?
One possible explanation would be that waves of heat is moving through the liquid which causes temporary change in refractive index at its location but this explanation is quite weird in the sense that thermal energy is moving in packets instead of being transferred continuously.