After shuffling the cards in a standard 52-card deck, Alex Mullen, a three-time world memory champion, can memorize their order in under 20 seconds. As he flips though the cards, he takes a mental walk through a house. At each point in his journey — the mailbox, front door, staircase and so on — he attaches a card. To recall the cards, he relives the trip. This technique, called “method of loci”…