Shapes fall from the sky, all you have to do is to control how they fall and fit within each other. A simple premise, but add an annoyingly addictive electronica soundtrack (based on a Russian folk tune called Korobeiniki, apparently) and you have a revolution in entertainment. Since Tetris was launched on the world in 1986, millions of hours have been lost through playing this simple game. Since then, we’ve seen games consoles grow in power, and with it the appearance of everything from Call of Duty to World of Warcraft. Yet block and puzzle games like Tetris still have a special place in our hearts. Why are they are so compelling? The writer Jeffrey Goldsmith was so obsessed with Tetris that he wrote a famous article asking if the game’s creator Alexey Pajitnov had invented “a pharmatronic?” – a video game with the potency of an addictive drug. Such is its mental pull, there’s even been the suggestion that the game might be able to prevent flashbacks in people with PTSD. I had my own Tetris phase, when I was a teenager, and spent more hours than I should have trying to align the falling blocks in rows. Recently, I started thinking about why games like Tetris are so compelling. My conclusion? It’s to do with a deep-seated psychological drive to tidy up. Many human games are basically ritualised tidying up. Snooker, or pool if you are non-British, is a good example. The first person makes a mess (the break) and then the players take turns in potting the balls into the pockets, in a vary particular order. Tetris adds a computer-powered engine to this basic scenario – not only must the player tidy up, but the computer keeps throwing extra blocks from the sky to add to the mess. It looks like a perfect example of a pointless exercise – a game that doesn't teach us anything useful, has no wider social or physical purpose, but which weirdly keeps us interested. There's a textbook psychological phenomenon called the Zeigarnik Effect, named after Russian psychologist Bluma Zeigarnik. In the 1930s, Zeigarnik was in a busy cafe and heard that the waiters had fantastic memories for orders – but only up until the orders had been delivered. They could remember the requests of a party of 12, but once the food and drink had hit the table they forgot about it instantly, and were unable to recall what had been so solid moments before. Zeigarnik gave her name to the whole class of problems where incomplete tasks stick in memory. The Zeigarnik Effect is also part of the reason why quiz shows are so compelling. You might not care about the year the British Broadcasting Corporation was founded or the percentage of the world's countries that have at least one McDonald's restaurant, but once someone has asked the question it becomes strangely irritating not to know the answer (1927 and 61%, by the way). The questions stick in the mind, unfinished until it is completed by the answer. Game theory Tetris holds our attention by continually creating unfinished tasks. Each action in the game allows us to solve part of the puzzle, filling up a row or rows completely so that they disappear, but is also just as likely to create new, unfinished work. A chain of these partial-solutions and newly triggered unsolved tasks can easily stretch to hours, each moment full of the same kind of satisfaction as scratching an itch. The other reason why Tetris works so well is that each unfinished task only appears at the same time as its potential solution – those blocks continuously fall from the sky, each one a problem and a potential solution. Tetris is a simple visual world, and solutions can immediately be tried out using the five control keys (move left, move right, rotate left, rotate right and drop – of course). Studies of Tetris players show that people prefer to rotate the blocks to see if they'll fit, rather than think about if they'll fit. Either method would work, of course, but Tetris creates a world where action is quicker than thought – and this is part of the key to why it is so absorbing. Unlike so much of life, Tetris makes an immediate connection between our insight into how we might solve a problem and the means to begin acting on it. The Zeigarnik Effect describes a phenomenon, but it doesn't really give any reason for why it happens. This is a common trick of psychologists, to pretend they solved a riddle of the human mind by giving it a name, when all they've done is invented an agreed upon name for the mystery rather than solved it. A plausible explanation for the existence of the Effect is that the mind is designed to reorganise around the pursuit of goals. If those goals are met, then the mind turns to something else. Trivia takes advantage of this goal orientation by frustrating us until it is satisfied. Tetris goes one step further, and creates a continual chain of frustration and satisfaction of goals. Like a clever parasite, Tetris takes advantage of the mind's basic pleasure in getting things done and uses it against us. We can go along with this, enjoying the short-term thrills in tidying up those blocks, even while a wiser, more reflective, part of us knows that the game is basically purposeless. But then all good games are, right?