The US and Japanese team found that the brain "doubles up" by simultaneously making two memories of events.
One is for the here-and-now and the other for a lifetime, they found.
It had been thought that all memories start as a short-term memory and are then slowly converted into a long-term one.
Experts said the findings were surprising, but also beautiful and convincing.
Two parts of the brain are heavily involved in remembering our personal experiences.