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We are surrounded in everyday life by yield stress fluids: materials that behave as solids under small stresses but flow like liquids beyond a critical stress. For example, paint must flow under the brush, but remain fixed in a vertical film despite the force of gravity. Food products (such as mayonnaise), other consumer products (such as toothpaste), concrete, and even radioactive nuclear waste sludge exhibit yield stresses. The yield stress may serve to keep particulate fillers from settling, as in many consumer products and gelled propellants, or determine whether bubbles remain trapped in cement. For handling and using these materials, it is paramount to know the stress at which the material starts to flow, but a consensus on the mechanical behavior of these materials is only slowly emerging.