Everybody knows that when you cool water to 0 °C (32 °F) it forms ice … except that in some cases it doesn’t! You can actually chill very pure water past its freezing point (at standard pressure, no cheating!) without it ever becoming solid.
Scientists know a lot about supercooling: it turns out that ice crystals need nucleation points to start forming. These nucleation points could be anything from gas bubbles to impurities to the rough surface of the container. Without these things, water would continue to be a “supercooled” liquid well below its freezing point.