Jon K replied: "The state that matter exists in (solid, liquid, or gas) is summarized by what's called a "phase diagram". Think of it as kind of a map of where the different states of a pure substance exist, except the two "directions" (axes) are pressure and temperature. If you're not changing the pressure, as would be the case if stuff were just sitting out, the only way you can move on the map is to higher or lower temperature. Usually to convert a gas to a liquid just means that you move to lower temperatures. But that doesn't always work. Some substances don't have a liquid state at normal pressures, so when you move down the temperature axis, you go directly from gas to solid. Carbon dioxide, the gas in the atmosphere that we and burning fuel put out and plants take in, is one such substance. Carbon dioxide goes directly from a solid ("dry ice") to a gas. To make liquid carbon dioxide, you need to not only move down the temperature axis, but up the pressure axis as well. Then as its phase diagram shows, you are in the region of liquid carbon dixide. For example, at 20 degrees C under a pressure of 56.3 atmospheres, carbon dioxide is a liquid, and that's how it exists in carbon dioxide fire extinguishers. When you use one of those fire extinguishers, you see a stream of carbon dixoxide "snow", the solid form. That's because as the liquid form evaporates, it cools down the gas, but now it's at normal atmospheric pressure so the gas goes directly into the splid state. You can follow this process on the phase diagram. So, the bottom line: you can make a gas turn into a liquid by the right combination of temperature and pressure, but sometimes you have to tweak both. Generally, you want lower temperature (the molecules become less active and clump together) and/or high pressure (you shove them together)."