“A glass is permanent, but only over a certain time scale. It’s a liquid that just stopped moving, stopped flowing,” said David Weitz, Mallinckrodt Professor of Physics and Applied Physics at Harvard’s School of Engineering and Applied Sciences (SEAS) and the Department of Physics. “A crystal has a very unique structure, a very ordered structure that repeats itself over and over. A glass never repeats itself. It wants to be a crystal but something is preventing it from being a crystal.”
-(exerpt from, http://news.harvard.edu/gazette/story/2009/11/new-wrinkle-in-old-approach/), http://www.news.harvard.edu/gazette/2006/12.14/16-glassanimals.html