What molecules or chemicals, other than water [H20] (that I know of), are less dense as they become cooler and their electrons slow? This is broad question, I know, but my question relates to the fact that much of our conditions of life rely on the density of water as it becomes a solid (see the last post at: http://www.pausd.palo-alto.ca.us/tosa/k6science/water/w_q_a/expand.html] by Jim Stringer). If there are other molecules or chemicals that react the same way then there could be other forms of life, besides Earth's terrestrial, outside of Earth so this is where my question originates from. I realize this may be a question of a green-horn, but I ask it nevertheless. I know there could be thousands, but I am interested in the simple molecules containing between 4 elements and 6 elements. Thank you.