Why can you only keep a few things in mind at once?

You can only keep a few things in mind at once because working memory, the mental workspace where you hold and juggle information for a few seconds, has a small, fixed-size desk. In 1956 the psychologist George Miller suggested a "magical number seven": young adults can hold about seven items, or "chunks". Later work refined it: about seven digits, six letters or five words, and some researchers, such as Cowan, put the true limit nearer four chunks.

The model most textbooks use comes from the psychologists Baddeley and Hitch in 1974. A "central executive" directs attention, helped by two assistants: a phonological loop that keeps sounds alive by repeating them, which is why you can hold a phone number as long as you keep muttering it, and a visuospatial sketchpad for images and mental maps.

The trick to beating the limit is the chunk. A chunk is the biggest meaningful unit you recognise: a word is one chunk if you speak the language, a jumble of sounds if you don't. With heavy training, some people have pushed their digit span to 80. One man studied by Anders Ericsson's team turned strings of digits into famous racing times. But his workspace hadn't grown: he'd learned to lean on long-term memory instead.

That's also why "brain training" is debated. Practice reliably improves the trained task, but the further you get from it, the weaker the benefit.