There’s something slightly mysterious about London cabbies. First off, those black cabs: sleek and unique on London roads. You see them flitting around London at any time of the day or night. They’re everywhere (well, except for when you need one).

But more than that, every cab driver has this knowingness to them. From behind their protective screen they’ll ask you where you want to go, with this understanding look when you ask to get to the cabbage seller just off Old Kent Road. There’ll be a slight pause then: “Ahhhh you mean Ted’s shop on Rowcross Street. Tell him I said hi.” And you’re off.

This isn’t just luck: becoming a cab driver in London is a lot like joining a cult. In fact, joining a cult would probably be easier.

An elephant's memory is nothing compared to our cab drivers

Since 1865, to become a cab driver in London you don’t just need to drive jerkingly or know your own batch of swear words, you have to pass The Knowledge. The Knowledge. Sounds like something that will only be given on the stroke of midnight by a cloaked figure, but actually it’s just a test where you (just) have to know all the roads in London of the top of your head. All 25,000 of them.

To make matters worse, if you take a glance at any map of London you’ll quickly realise that this is not a city that was planned out. Unlike New York or Singapore’s neatly gridded streets, London is, in a word, a mess. Road becomes side roads become dead ends and then become main roads again. Going from A to B is like trying to work your way through a maze. Try to plan out now how to drive from the London Eye to St Paul’s Cathedral. Two major landmarks, but not quite so easy, even if you do have a map? Now how about doing that in rush hour from memory and being able to name all of the roads you take.

That’s what you have to be able to do to pass The Knowledge. It’s incredibly difficult. Most people take two years memorising it all, because not only do you need to know all the roads, but you also need to know how to use them to get as quickly as possible from any road but also from any of London’s 20,000 landmarks. It’s an insanely tough test of your brain. And this learning process changes you forever.

MRI? Or London from the sky?

Gaining The Knowledge is such an intense experience that cabbies’ brains are fundamentally altered:  Eleanor Maguire from University College London has shown in her research that cabbies have a different hippocampus, the area in your brain in charge of memory and spatial reasoning, from the rest of us. This means that they can not only remember more than you and me, but they’re better at plotting out what they know.

So the next time you jump in a cab and ask to get to Piccadilly as quickly as possible, spare a thought for not only how you’re being taken there by a trained professional, but also one that’s taken the most unbelievably quirky and tough training. Maybe even ask him about the Knowledge.