Get into their head - Idioms by The Free Dictionary
get (something) into (one's) head (redirected from get into their head)
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.