The Zeigarnik effect states that open tasks (either interrupted or unfinished) tend to occupy people’s short-term memory until they are done.
We can use the Zeigarnik effect to our advantage. If you want something to stop intruding your mind, write it down and promise yourself that you’ll “deal with it later”. If you want to keep pondering something (perhaps a problem you want to solve), don’t write it down, and go for a walk with that problem on your mind.