If you heat water slowly, almost everything about the process seems gradual. The temperature creeps upward, molecules move faster, and for a while there’s no obvious reason to expect the system to suddenly become a different kind of thing. Then you cross the right combination of temperature and pressure and the behavior changes much more sharply than the buildup suggested it would. That kind of transition shows up all over science. A system can change little by little for a long time, then cross some boundary where a new pattern becomes possible and the old description stops being enough. I find that weird because the ingredients usually haven’t changed. The water is still made of the same molecules it had a moment earlier. What changes is the way those molecules behave together, and once enough of them are interacting under the right conditions, the large-scale system starts having properties that don’t really belong to any one molecule on its own. So why does nature seem to have so m...