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."
get something into your/somebody’s (thick) ˈhead (informal) succeed in understanding or in making somebody understand something fully: When are you going to get it into your thick head that you don’t need to worry about money? You’re rich now. ♢ I’m still trying to get it into my head that I’m free to do what I want now.
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.