When you freeze water, it expands -- this is an important and unusual property of water, since most substances are denser as solids than as liquids, and so a given weight of them is smaller when it solidifies. In fact, this tendency to expand is remarkably forceful, and if you try to freeze water in a box which doesn't give it room to expand, you're going to need a new box.
No, really. This article tells you about some of the experiments people tried with this (e.g., sealing them in artillery shells; when the water finally froze, their cast-iron plugs ended up flying away at remarkable speed). The reason it doesn't, ultimately, work, is that ordinary ice -- what's technically known as "Ice-1" -- is just plain bigger than water, and the only way to make it smaller is to put enough pressure on it that the molecular structure of the ice changes. At a pressure of 1,736 atmospheres, it transforms into "Ice-II," which is denser than water: instead of forming hexagonal cells (the same hexagons which lead ordinary ice to form hexagonal snowflakes), the water molecules now arrange themselves into slanted rectangular prisms.
It turns out that ice actually has more than a dozen such forms, each with different physical and chemical properties; Ice-XV, for example, resists holding any kind of electrical charge, and the as-yet-uncreated (and unnamed) forms of ice above a pressure of 10 million atmospheres would be metallic.
h/t +Alex Scrivener
and +Keith Wilson
for the link.