Categorization

Categorization is the fact that people recognize similarities of different things, and put these things into groups, called categories. Categorization is one of the most fundamental cognitive abilities. As such it is studied by psychologists and people interested in behaviour. People form these categories either because of other things they know (pre-existing knowledge), or by comparing them with other similar entities. 
While the categories use in science rarely have exceptions, categories in everyday life often do. 
As an example: Neither Rheas nor ostriches can fly, yet even children will classify them as birds. 
Today, scientists tell us that other animals, such as the great apes, horses or even doves have the ability to form categories.