useful in solar installations because it absorbs so much energy. That energy can be released when the gas condenses back into a liquid, and we can gather it to heat water, for example.

Now here are the oddities: most liquids get more dense and compact as they get colder. It's usually a linear thing – the colder they are, the more dense and compact. But water is different.

Water is at its most dense when it reaches 39°F (4°C). Once it gets colder than that, water starts getting lighter. Frozen water is light and floats, whereas slightly warmer water sinks. This is, in fact, one of the main reasons why there is life on earth: the bottom of a frozen lake is always liquid, so the fish, algae and other stuff can still live, even though the surface is solid.

At the bottom end of the scale, water freezes at 32°F (0°C). When it freezes, water expands by 9%. That may not sound like a lot, but it's huge compared to most liquids, and it's catastrophic for closed circuits such as pipes and storage tanks. Here's a look at what happens when water freezes inside a pipe:

http://www.youtube.com/watch?v=HFMJp2xaKCk