get/take it into your head that...

get (something) into (one's) head

To believe that something is true. Get it into your head: I'm in charge until Mom gets home, so you have to listen to me. A: "Why is Johnny holding his breath?" B: "Well, he's gotten it into his head that holding his breath will make him taller."
See also: get, head

get/take it into your ˈhead that...

(informal) think or believe something, often wrongly: Somehow she’s taken it into her head that her husband is trying to poison her.
See also: get, head, take