Language is a social energy, and our capacity for articulate speech is the key factor that makes us different from other species. We are not as fast as cheetahs – or even as horses. Nor are we as strong as bulls or as adaptable as bacteria. But our brains are equipped with the facility to produce and process speech, and we are capable of abstract thought. A bee may dance to show other bees the location of a source of food, a green monkey may deliver sophisticated vocal signals, and a sparrow may manage as many as thirteen different types of song, but an animal's system of communication has a limited repertoire; ours, on the other hand, is 'open', and its mechanisms permit a potentially infinite variety of utterances.