Mispelled

When the water loses energy, the chemical forces (hydrogen bonds in this case) become strong enough to keep the molecules from bumping all around. This forces them to settle into a crystal lattice where each O atom has two H atoms from other molecules next to it. This takes up more space than if the molecules were more randomly scattered, as in liquid state. I found some good pictures here: http://www.iapws.org/faq1/freeze.htm So, to answer your question, it is the hydrogen bonding that causes the expansion.