Video: When you rub your hands together to create friction and warmth, heat energy radiating off your hand creates air currents. They're completely invisible to the human eye, but with a simple setup, it turns out your digital camera can reveal this invisible world around us.
The technique is known as Schlieren photography, which YouTube's Veritasium — AKA Derek Muller — was able to recreate using a DSLR, a small concave mirror, a tiny LED bulb and a razor blade. His exact setup is explained in the video, but essentially, it allows the camera to see dark and bright spots created by changes in light's index of refraction as photons travel from the LED, to the mirror, and back to the camera's lens.
As light travels through air of different temperatures and compositions, it ever so slightly changes speed and bends (remember the classic "crooked pencil in a cup of water" experiment from primary school?), which result in distortions. These distortions are so subtle the human eye can't seem them, but they're amplified using this clever setup so a camera can — revealing a tumultuous world that, quite frankly, we're happy to be oblivious to.