The ability to use language (to communicate using symbols and words to represent objects, feelings and thoughts) separates humans from all other animals on earth. By the age of 18, humans know more than 60,000 words, which is a surprising figure when you consider the monosyllabic vocabulary used by most teenagers. From the age of six months babies begin to recognise their own language. There are genetic reasons why this form of communication is unique to humans. Humans and chimps actually share 99% of their DNA, but in 2006 researchers comparing the two species found a gene, HAR1, that makes us dramatically different. It is expressed in the human cerebral cortex, a part of the brain that is particularly associated with language.