YouTube user magx1 builds a lot of home-made gas lasers, running electric current through sealed tubes filled with various gases. When you do that, you get the gas atoms all riled up and their electrons absorb energy. Eventually, that energy gets released in the form of directional, highly focused light—a laser. There's a bit more to it than that, but you've got the idea.
One type of laser magx1 builds is called a carbon dioxide laser, though it actually contains a mixture of carbon dioxide, nitrogen, hydrogen and helium. In this video, the carbon dioxide in the mix is coming from his own breath.
By exhaling into my 'supply' balloon, I was able to get pretty good results considering! Unlike a 'proper' laser mix of CO2, N2, and He; the 'breath' trick appears to have a much narrower pressure range over which lasing will occur. But I like it anyway - it doesn't get much more DIY than this!
Magx1 doesn't provide a lot of detail on how he built this laser, but you can find a lot of good information at The Home-Built Carbon Dioxide Laser FAQ. Bill Beatty—who suggested this video through the Submitterator—says Magx1 is a guy named Jarrod Kinsey, and that he has a site dedicated to laser building.