As the hydrogen and oxygen molecules that make up water begin to transfer their heat to the freezing air around them, they bind more tightly. If the pattern of the hydrogen and oxygen molecules remained unchanged, all this huddling would result in solid, frozen matter that would actually take up less space than the liquid form. But water just has to be different. Instead, the pattern of its molecules changes as it freezes, taking up more space and expanding as it turns into ice [source: The Physics Van].
And when it comes to the water pipes in your home, this expansion of water as it freezes can put a lot of stress on stiff metal and plastic pipes, as well as pipe joints. Take note, inventors: If water pipes were made out of an expandable material, freezing water wouldn't affect their performance and would eliminate damage to thousands of homes each year.
So this means that when water expands into ice it causes pipes to burst, right? Not directly. Instead, the problem occurs when a pipe becomes blocked by ice, often in multiple locations within the pipe. These blockages -- located between the water source and the closed faucet -- cause a buildup in water pressure. Upstream from the ice blockage, the water can retreat back to the source without pressure buildup. Downstream from the ice blockage, the water is trapped between the blockage and the closed faucet. If more ice forms, it has nowhere to go and the pipe will burst. Pipe joints are particularly susceptible to leaks, but cracks can occur along other sections of a pipe, too [sources: The Weather Channel, The Physics Van].
This is one reason why it's a good idea to leave a faucet slightly open if you're worried your pipes may freeze. Some people think the constant drip keeps the water from freezing but actually water can freeze while even flowing. The real benefit is relieving the pressure [source: The Weather Channel].
Although pipes will burst or begin to leak as they thaw, leading some to believe the melting process causes problems, that's not the case. The actual damage will have occurred days earlier when the pipes became frozen; you simply won't know how much damage was done until the thaw begins because the ice blockage had been acting as a plug.