Knowledge boy David Greengalgh tries to looks under the door of the Apothecaries Hall,  as he does a pointing run of the Livery Halls in the City of London Sunday, June 22, 2014. ‘The Knowledge’ is a 150 year-old London tradition, so called because of the encyclopaedic  knowledge an aspiring London cabbie is expected to have of the city and its (now) 25,000 streets and the 20,000 or so landmarks that lie within a 6-mile radius of Charing Cross Station before he/she can become a licensed black cab driver. It has been around since 1865 and the days of hansom cabs. It is an extremely difficult test to pass – easily the toughest exam faced by any cab driver anywhere in the world. The failure rate is 70%. (Elizabeth Dalziel for National Geographic)