Alexa- That's a very nice question.
The answer is that a single water molecule can't change from liquid to solid or gas. Those are names of ways that large batches of water molecules behave, not single molecules.
For example, the solid (ice) has a batch of molecules stuck together lined up in a regular pattern. A single molecule can't do that by itself.
The liquid is a batch of molecules that are stuck to each other but not in a regular pattern, so they can still tumble around. The gas has the molecules flying around so much that they mostly quit touching each other.
What causes the big batch of molecules to change between liquid, gas, and solid? The amount of energy around per molecule is the key. If there's little energy per molecule they freeze into a solid. We say they're cold, or at a low temperature. If there's a lot of energy per molecule they break loose and form a gas. We say they're hot or at high temperature. In between, they form a liquid.
(published on 10/03/2011)