to use one's brains; to think carefully or actively; to rack one's brains

knotty; bothersome; difficult to solve (of a problem or situation)

rigid in thought or behavior; refusing to adapt to circumstances

To update one's perspective by replacing old ideas or habits with new ones; to view a situation from a different angle.

outdated or old-fashioned way of thinking; a stubborn mentality that is resistant to change

a riddle or game that requires unconventional or humorous thinking to arrive at an unexpected answer

idiot; porridge head; someone whose brain is like bean dregs