Have you ever seen or heard something amazing – a scene in a film, a joke or a song – only to forget it later on? Instead of the crystal clear images you wanted to recall, you’re instead left with scraps of images and mangled sentences, or more frustratingly still, nothing at all. Even monumental events, like meeting a film star, can sometimes fade surprisingly quickly.

There may be a disarmingly simple way to cement those memories, however. According to research by Chris Bird at the University of Sussex, all it requires is a few seconds of your time and a bit of imagination.

Bird recently asked some students to lie in a brain scanner and view a series of short clips from YouTube (involving, for example, neighbours playing practical jokes on each other). Straight after some of the clips, they were given 40 seconds to replay the scene in their minds and describe it to themselves. For the others, they just moved onto a new video.

By simply describing the event to themselves, they were able to remember twice as many details a week or two later

It turned out that simply describing the event to themselves massively improved their chances of remembering it accurately a week or so later: on average, they were able to remember twice as many details. Want to prove it for yourself? Take a look at the short video below to test this simple principle of memory improvement, and you will see how powerful it can be.

Bird also found that his brain scans appeared to reflect the strength of the memory: when the activity during their descriptions closely mirrored the activation as they watched the video itself, the students seemed to have built particularly strong foundations for later recall.

That may, perhaps, be a sign of just how much effort and detail they were imagining as they described the scene. It could also be that it allowed the students to peg the events to other memories; one student compared a character in the clips to James Bond, for instance – instantly making him more memorable.

In other words, if you want to make sure something sticks in your mind, just take a minute or so to describe it to yourself, consciously and deliberately picking the most vivid details.

Bird can see how it might be particularly important in the courtroom. “The findings have implications for any situation where accurate recall of an event is critical, such as witnessing an accident or crime,” he says. “Memory for the event will be significantly improved if the witness rehearses the sequence of events as soon as possible afterwards.” But it could be equally helpful for anyone hoping to cling to something worth remembering.

Interested to learn more ways to boost your memory? Here are BBC Future's guides on "How to learn like a memory champion" and "How to learn 30 languages" .

David Robson is BBC Future’s feature writer. He is @d_a_robson on twitter.

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