Over the past few decades we have learned a great deal about the behavior of such materials as liquid crystals, emulsions and colloids, polymers, and complex molecules. These materials, called "soft matter" ("matiere fragile" in French), have neither the rigid structure and crystalline symmetry of a solid nor the uniformity and disorder of a fluid or a gas. They have unusual and fascinating properties: some change their viscosity at our beck and call; others form layers of two-dimensional liquids; some are polarized, their molecules all oriented in the same direction and turning in unison at...