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" ("mati re 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;...