Why does hearing an old ABBA ditty remind you of your prom night? Or why the aroma of 'fish 'n' chips wafting in bring to mind that sunny picnic few years ago?
Now, scientists from the National Institute of Neuroscience in Turin tell you why.
Studies have shown that something we smell or hear is woven deeply within our memories at that particular time, place or person.
For example, the animals that had seen a flashing light while being shocked held the memory of the shock in a brain region that processes visual data.
It is thought the two visual and aural memories are woven together in the grey matters - meaning that retrieving one also triggers the other, reports the Daily Mail.
The finding explains why a whiff of a certain perfume reminds us of the only person who used it.
Just as the whirr of a computer modem is for many an instant reminder of the Nineties.
But our recollections are not always accurate. Scientists believe our memory wears rose-tinted glasses, remembering the good and forgetting the bad.
Studies have found that no matter how badly we have been hurt in the past, we dwell on the positive when thinking back.