Everybody loves dry ice. It's mysterious, and sexy, and makes your whole life look like a 1980s music video. But it's also expensive, and it's generally only made in industrial facilities. The good news is, if you're willing to go to a relatively large amount of trouble, you can make it at home.
Just make sure that you've got extra fire extinguishers (though not for the reason you might think!)
I cannot tell you how gratifying it is to finally be able to recommend an experiment that involves the use of a fire extinguisher, but doesn't present any threat of fire. Instead, I'll tell you a little about dry ice. Dry ice isn't actually made of water — it's carbon dioxide cooled until it forms a solid. Because it doesn't exist at room temperature and pressure as a liquid, it doesn't melt and get wet carbon dioxide all over the rug. Instead it goes straight to gas form and blows away in a dramatic swirling mist.
The only people who can easily make dry ice are those who have access to a lot of carbon dioxide: generally manufacturers. Carbon dioxide gas is compressed until it forms a liquid. The pressure is then released as the carbon dioxide is pumped into a large empty chamber. The carbon dioxide immediately turns into a gas and expands. As it expands it cools down rapidly. (The process of cooling a substance by suddenly allowing it expand is called adiabatic cooling and is what cools down your fridge, as well.) The drop in temperature is so extreme that some of the CO2 cools enough to become solid, and it snows carbon dioxide. The snow is then pressed into ice cubes.
To make your own, you need a lot of CO2 — and no, breathing in and then breathing out into a plastic bag won't be enough. A CO2 fire extinguisher, however, will provide the necessary amount. But you'll need to have another extinguisher as a replacement, in case (unrelated to this experiment) there is a fire somewhere in the area. Never let it be said that io9 neglects your personal safety! As a further safety precaution, put on heavy gloves, because this process is very cold. Tape a cloth bag around the nozzle of the fire extinguisher and shoot the fire extinguisher into the bag. The extinguisher discharge works in much the same way the adiabatic stage of the dry ice manufacturing process works. Ice will begin forming in the bag, and you can press it together and play with it at your leisure!
Top Image: Shawn Henning