Previously, previously, previously, previously, previously.
Does it look like a bitch?
Oh, man, this takes me back. When I was a kid, my father had a bookshelf full of back issues of Scientific American from the 50s and 60s in my closet. The Amateur Scientist column was my favorite, they had several pieces about people who'd built their own Schlieren photography apparatus.
I've done Schlieren photography, it's fun. You can get a cheap 6" spherical front surface mirror off eBay for a few bucks, that's all it really takes.
Even simpler, you can do a shadograph by luck in your apartment or bar. Some time when the sun is in the right place and a car is parked in the right place, the sun will reflect off the convex windshield and create a slightly odd looking anti-shadow inside the building, often on the ceiling. That reflected light is near a point source, and any changes in density will create shadows. So stick a lighter in the beam of light and light it, and you can see the low density gasses rising from the shadow of the lighter in the sunbeam's projection on the ceiling.
In the closing credits.... are those the two guys from The Electric Company?