A University of Sheffield psychologist explains the mystery behind tetris addiction on its 30th birthday.

June 6 is also known as World Tetris Day. People have spent millions of hours trying to fit falling shapes into rows. Tetris is considered one of the biggest-selling computer games of all time.

Dr Tom Stafford from the Department of Psychology said that the human mind likes to tidy up and the game gives the brains the basic pleasure of finishing up perpetually uncompleted tasks. The prospect of solving more mini tasks is as satisfying as scratching an itch and that keeps users endlessly playing.

"Tetris is the granddaddy of puzzle games like Candy Crush saga - the things that keep us puzzling away for hours, days and weeks. Tetris is pure game: there is no benefit to it, nothing to learn, no social or physical consequence. It is almost completely pointless, but keeps us coming back for more," Stafford said in a statement.

The simple and irresistible game is described as a "pharmatronic" - software programs that induce drug like effects in the end user, Termwiki reports. The Tetris effect is so significant that players see falling shapes in their mind's eye even after they finish playing.

Tetris also has the "Zeigarnik effect", the psychological tendency to remember an uncompleted task rather than a completed one.

"Tetris does this wonderfully...it presents a world of perpetual uncompleted tasks," Stafford said. "It shows how our minds are organised around goals and that our memory is not just a filing system where information is passively stored, but it adjusts dynamically according to our purposes," Daily Mail UK reports.

Previous research have shown that there are two ways of solving the Tetris puzzle - either by mentally rotating the brick in the mind's eye to see how it might fit into the line of blocks on the bottom or by quickly rotating the block on the screen to make the visual comparison.

The study found that most people prefer to use the key that quickly rotates the blocks in different angles to determine where to best fit them instead of using their mental energy.

Tags Psychologist, Addiction, Computer