Paint by numbers

When it comes to remembering numbers or binary digits, each competitor has their own system for converting these items into images. Mullen, for instance, uses a "two-card" system for memorising a pack of cards. It involves converting suits and numbers in phonetic sounds. Take the seven of diamonds and the five of spades, for example. The diamond and the spade together create an 'm' sound, he says. The seven is converted to a 'k' sound and the five is an 'l'. Although it's not immediately obvious where each sound comes from, they are in fact based on a code that someone else thought up a long time ago, says Mullen. "Once you've got your m, k and l, you just make up an image which fits those letters - they reminded me of the word Michael, so when I see that pair of cards I think of Michael Jordan." He says he has another trick which halves the 2,704 possible pair combinations, but he didn't want to go into details.

Mullen won the world title from Jonas Von Essen. Like Mullen, Von Essen also discovered a love of memorising things late in life. "I was wandering around a library and saw a book that said you can improve your memory and thought it sounded cool, because I really wanted a better memory so that I could be better at school and exams," he says.

Almost immediately I realised I could memorise more things than I ever dreamed possible

Von Essen, a student at the University of Gothenburg, tried out a few of the techniques - the benefits were instantaneous: "Almost immediately I realised I could memorise more things than I ever dreamed possible," he says. His first competition was the Swedish memory championship in 2012, which he won. He went on to become world memory champion in 2013 and 2014.

Von Essen uses a slightly different technique to remember cards. He has an image that he associates with each individual card, which he groups into sets of three, before placing them on a short walk through his house. "So I might start at the front door," he says. "Say it's the four of hearts, the nine of hearts and the eight of clubs - I'd open the front door and see an image of Sherlock Holmes playing the guitar and eating a hamburger. Then I'd walk into the hall and create a new scene with the next three cards."

What's your favourite restaurant?

Memory palaces can be anywhere - hotels, houses, routes to work, restaurants, a favourite holiday, a park or a train ride. "If you sit and think about it for a moment you can come up with hundreds of locations that you know pretty well," says Mullen.

Both of the world champions use specific mind palaces for things they want to remember in the short term - like a pack of cards - and things they want to remember forever. "So for the five-minute number competition I use the same two palaces, then re-use them the next time," says Mullen. "Because I don't want to remember a five-minute event I did two years ago. But when I'm learning things for school, say drugs that treat stomach disorders, I'll fill up a palace and not use it for anything other than related information."

'Does it work every time? Doesn't your mind ever go blank?' I asked both competitors. No, was the answer. "If you've put it in your mind palace, it's always safe," says Von Essen.

Another thing they agree on is that the gift of extraordinary memory is nothing special - anyone can learn how to do it to a fairly decent standard. Mullen trained just half-an-hour to an hour each day in preparation for the World Championships. Von Essen also trained for small amounts of time each day, before gearing up to five hours a day just before competitions.

Von Essen has taken a break from competing, but he's hoping to break a world record next year for memorising pi. So far he's reached 10,000 digits, but by next year, he hopes to have increased that to 100,000 digits.

Mullen can't make a living out of his title. "There's prize money but not enough to live on," he says, so is instead concentrating on using his techniques to help him - and others - through school. "I do my best to promote the techniques to others because they are really useful for everyday life. I'm trying to show people how to use the techniques for learning more generally, rather than just for competitions," he says. "I like the whole competitive process, but I think there's a lot more you can gain from using these techniques in different areas of your life."

Von Essen agrees. It's like riding a bike, he says. It seems difficult and impressive if you haven't tried it, but with a bit of practice, anyone can do it. "You don't have to become a world champion to have a much better memory than normal," he says, "same as you don't have to win the Tour de France to get to the store faster than walking."