Nothing in science - or the world, really - is a simple as you learned at school. Take water. If you thought all you had to do to turn water into ice was cool it below zero degrees and wait, you would be wrong.
In fact the key process is called ice nucleation, by which ice forms around something, whether it's a protein or bacteria. Using bacteria or additives, it's possible to freeze water into ice at far higher temperatures than you might think - and indeed a version of this was used to create the 'fake' snow for the winter Olympics in Sochi.
This video shows that process happening in a pretty extreme case. Bacteria is added to supercooled water, and the result is instant, total ice.
"In my Microbiology course at the University of Puget Sound, I like to demonstrate the wild and wonderful and weird microbial world. In this video, I supercool a bottle of water to approximately -6 degrees C. I then a drop of a Pseudomonas syringae culture. This organism makes an ice nucleation protein, which allows the supercooled water to immediately change to water ice...while you watch."