Ling Liu

Drop two pebbles into a pond: each sends out ripples, and on the water you'll see crests where waves reinforce each other and troughs where they cancel. Protons behave much the same way: when there are two, they create a double ripple that adds up into a recognizable pattern.

Imagine a slide with three dips: the deepest one is a stable position for a ball, the middle one is precarious. A slight push and the ball rolls down. Yet, constant vibration of the right strength can hold it in the middle hollow for a long time. In the 1950s, physicists Lee and Yang discovered that all states of matter are separated by invisible points in the world of imaginary numbers.

If you squeeze water between two glass plates so that only two layers of molecules fit, it becomes something between a liquid and ice. At room temperature, molecules in such a trap still move but arrange themselves into a semblance of a crystal lattice.