The three states of matter are well represented by earth, sea, and sky. God saw that it was good, and so did many generations of physics lecturers. But purity is obscurity, as Ogden Nash reminded us. The rich variety of combinations of gas, liquid, and solid that are found in nature’s creations and mankind’s artifacts presents us with an infinity, rather than a trinity, of possibilities (see figure 1).

Today, solid-state physics has extended its horizons to encompass all of condensed matter and the wide world of materials science. Physicists share with physical chemists and industrial engineers an eclectic interest in the exceptional properties that can emerge from mixed phases. For example, ice cream—consisting of crystalline solids, liquid, and gas—is much more than the sum of its parts. Offered separately, those parts might provide the same nutritional value, but the pleasure of consumption would be lost. Here, structure is...

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