When we have unfinished tasks, we think about them continuously. But the moment they are completed, we forget about them. If we have unread email, we constantly wonder what it says. But once it has been dealt with, we often cannot recall the details of it.
The name for this phenomena is called the Zeigarnik effect and named after Russian psychologist Bluma Zeigarnik.
Once our brain receives information, it temporarily stores sensory memory (sight, hearing, smells, taste, and touch). If we pay attention to the information, it moves to our short-term memories.
If the task is incomplete, our brains can't let it go until it's done. That is why TV dramas use cliffhangers to end episodes.