The Tetris effect occurs when people devote sufficient time and attention to an activity that it begins to overshadow their thoughts, mental images, and dreams. It is named after the video game Tetris. People who play Tetris for a prolonged amount of time may then find themselves thinking about ways different shapes in the real world can fit together, such as the boxes on a supermarket shelf or the buildings on a street. In this sense, the Tetris effect is a form of habit. They might also see images of falling Tetris shapes at the edges of their visual fields or when they close their eyes. In this sense, the Tetris effect is a form of hallucination. They might also dream about falling Tetris shapes when drifting off to sleep.