Drop an ice cube into a glass of water and it floats. This seems normal, but it is actually quite unusual. Most substances become denser and sink when they turn solid — yet water does the opposite.
When water cools down and freezes, its molecules arrange themselves into an open, six-sided pattern. This pattern takes up more space than liquid water, so the same amount of water becomes slightly larger and lighter as ice. Because ice is less dense than the liquid around it, it rises to the top.
This behaviour matters far beyond a glass of water. In cold weather, the surface of a lake freezes first, forming a layer of ice that acts like a blanket. The water underneath stays liquid and a little warmer, allowing fish, plants and other creatures to survive the winter. If ice sank instead, lakes and rivers could freeze solid from the bottom up, and life in them would struggle to continue.
