‘What is an animal?’ outlines the criteria for defining an ‘animal’. There are characteristics by which we recognize animals — they are multicellular, get the energy necessary for life by eating other organisms, and can move and sense their environment — but these do not define an animal. In taxonomy, names are given to branches on evolutionary trees. Real or ‘natural’ groupings must encompass sets of organisms that have a shared evolutionary ancestor. This means that the term ‘animal’ must refer to a group of related species. The animals, therefore, are a natural group (or clade) descended from a shared common ancestor. This clade is called the Animal Kingdom, or Metazoa.