" The tipping model is a special case—a broad class of special cases—of critical-mass phenomena. Its characteristics are usually that people have very different cross-over points; that the behavior involves place of residence or work or recreation or, in general, being someplace rather than doing something; that the critical numbers relate to two or more distinct groups, and each group may be separately tipping out or tipping "
― Thomas C. Schelling , Micromotives and Macrobehavior