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?