Answer:
Loss of energy to a colder material. Ice forms at 32°F /0° C.
Step-by-step explanation:
As water cools, the natural movement between the molecules slows until it changes to a solid state (just as it boils to gas if they become too active). This movement doesn't completely stop until the material hits "absolute zero" at -460°F /-273° C. (0° Kelvin).
That's about the temperature "in the shade" in outer space.
A more interesting question would be, "why does water expand when it freezes?" The H - two - O molecule forms a triangle (with the O in the middle).
As it cools, the angle formed by the two H's gets larger. At around 36°F /2° C , the shrinking space between molecules gets overwhelmed by the expanding size of each molecule.
So, for those last few degrees before freezing, the overall volume gets bigger instead of continuing to decrease.
That's what causes potholes in roads (water settles into cracks, then makes the crack larger as it freezes). It's what damages the eaves trough around your roof, breaks leaves down into topsoil and splits boulders in half.