Stephen Morris, professor of experimental nonlinear physics at the University of Toronto, maintains a Web page on just this topic. So we asked him to tackle the question:| Scientific American
When water freezes, its molecules get arranged in crystalline structure, thereby attaining a defined shape. This crystalline structure is less dense and since there are gaps between individual molecules in the structure, the overall volume increases and water ‘expands’.| ScienceABC
Got Spikes| www.physics.utoronto.ca