Never thought of that but it makes sense. I have also heard that some water will freeze from bottom to top.
http://www.damninteresting.net/conte..._water_ice.jpg
Here, we can see ice that actually sinks to the bottom instead of floating. This water is called heavy water. Water is made up of 2 Hydrogens and one Oxygen. What we normally think of Hydrogen contains 1 proton and 1 electron. In heavy water, the hydrogen contains 1 neutron, 1 proton, and 1 electron. This isotope is called Deuterium and is much more dense than regular Hydrogen.