Our days are full of things to remember, and they don't always arrive in an orderly fashion. Perhaps you begin your commute home and remember that you need to pick up milk. But then immediately, another to-do springs to mind: You never called back your friend last week. You may try to hold both in your head, but in the end the milk, the phone call or both still sometimes fall away, forgotten.
A new scientific model of forgetting is taking shape, which suggests keeping multiple memories or tasks in mind simultaneously can actually erode them.