If you’ve ever seen heat mirages, where the road ahead waves a bit on a very hot day, you’ve gotten a hint of Schlieren imaging, which is a way of seeing the flow of otherwise invisible gases.
When waves propagate from one medium into another, the waves bend a bit due to changes in the index of refraction of each material. This is why a straw in a glass of water appears oddly shifted: light goes from air to water, which each have different indices of refraction, and the path of the light is altered, which makes it look like the light reflecting off the straw is coming from somewhere else.
Now if you take those light waves and ‘straighten them out’ so they’re all moving in parallel, any alterations in those waves will be more easily visible, and that’s what you see in the video above: heat from various objects flowing and dancing just like an invisible (and gravity-defying) liquid. If you happen to have a parabolic mirror handy (who doesn’t?) and want to try it yourself, have a look at this:
Recently, NASA researchers also figured out a way to use this technique to actually see the shockwaves produced by supersonic jets crossing the sound barrier!