* Most substances DO contract when frozen: This is because the molecules slow down and get closer together in the solid state. This is why ice floats on water, as ice is less dense than liquid water.
* Some substances EXPAND when frozen: This is because the molecules arrange themselves in a more open, less dense structure in the solid state. A common example is water.
So, while it's a common misconception that all solids contract when frozen, it's important to remember that there are exceptions!
