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Regarded as one of the toughest memory and navigational exams in the world, the Knowledge is a test that every London black cab driver has to get through. Since it began 160 years ago, it has thwarted and frustrated prospective drivers and takes roughly three to four years to pass, sometimes more. Speak to a black cab driver about it and you will detect a mix of fear, dread, relief and overwhelming pride that they passed. So how much of London do you need to memorise and does it really change the shape of your brain? Ross Ryan has more. (Image credit: Getty Images)