Animal cognition is that branch of the biological sciences that studies how animals perceive, learn about, remember, understand, and respond to the world in which they live. The mechanisms it investigates are evolved ways of processing information that promote the fitness or survival of the organisms that possess them. Interesting new findings about animal cognition have been revealed as part of the cognitive revolutions in psychology of the last 30 years. Among these are insights into how animals keep track of time, how they process numerical information, and how they navigate through space. Studies of time representation show that animals have one clock that keeps track of the time of day and another clock that times intervals as short as a few seconds. Other research indicates that animals can accurately estimate numbers of events and perhaps even summate numbers of objects and symbols that stand for numbers. Experiments on spatial representation have shown that animals use egocentric and allocentric cues to navigate from place to place. Egocentric cues are internal and are used for dead reckoning return vectors to a home base; allocentric cues are external cues, such as an environmental framework or landmarks, that are used by animals to remember and find important locations in space. Animal cognition studies also have revealed evidence for some advanced processes. As examples, pigeons show conceptual ability by learning to sort photographs into categories of people, flowers, cars, and chairs, and chimpanzees seem to reveal a theory of mind by behaving as if they understand the contents of other chimpanzees' minds.