It is a feature of all languages that novel utterances are continually being created. A child learning is especially active in forming and producing utterances with he or she has never heard before. With adults, new situations arise or new objects have to be described, so the language-users manipulate their linguistic resources to produce new expressions and new sentences. This property of human language has been termed productivity (or ‘creativity, or ‘open-endedness’). It is an aspect of language which is linked to the fact that the potential number of utterances in any human language is infinite.