Recalling a particular memory can cause us to forget another, similar memory - and neuroscientists have now watched this process happen using brain scans.
Inside the brains of human subjects, they pinpointed the unique imprints of two visual memories that were triggered by the same word.
Then they watched as repeatedly recalling one of the images caused the second, interfering memory to vanish.
The study is published in the journal Nature Neuroscience.
The results suggest that our brains actively delete memories that might distract us from the task at hand.
"People are used to thinking of forgetting as something passive," said lead author Dr Maria Wimber from the University of Birmingham.
"Our research reveals that people are more engaged than they realise in shaping what they remember of their lives."