Abstract: Sometimes single observations, such as the sighting of an animal thought to be extinct, or a chimpanzee making tools, are enough to change forever the way people think about the world. This chapter explores the situations when single observations are enough to destroy a hypothesis and the situations in which they are not. By emphasizing the power of the single unexpected or quantitative observation, the chapter paradoxically paves the way for an understanding of why, in most situations, repeated observations with large sample sizes are needed.

Keywords: coelocanth, tool use, extinction, unexpected observation, testing hypotheses,