As water freezes, its molecular structure changes to a hexagonal shape. This shape takes up more space than the molecules in liquid form but the change alone is not the reason pipes burst. What happens is that built up ice pushes up against the liquid water and blocks it from flowing. The water behind the ice can always push back to its source but water in front of the ice has nowhere to go – especially if your faucet isn’t open. Eventually the pressure will burst your pipe in an area that wasn’t actually frozen.