Something which everyone knows, if only in a vague way through personal experience or personal observation.
So why do a study on "stupid stuff like this?"
- Objective Knowledge. There's stuff we know, and there's stuff we both know and can back up. We're more assertive and certain about knowledge we can back up.
- If it turns out what everyone knows is wrong, then it's time to change what you know, or at least adjust it.
(although you got to admit, it's more interesting when "Everything You Know Is Wrong!")