use (one's) head
To use one's own intelligence and intellectual ability; to think logically and rationally. In this phrase, "head" refers to one's brain. Often said as an imperative. Come on, Dean, I know you can figure this out on your own. Use your head for a change!
use one's headand use one's noggin; use one's noodle
Fig. to use one's own intelligence. (The words noggin and noodle are slang terms for "head.") You can do better in math if you'll just use your head. Jane uses her noggin and gets things done correctly and on time. Yes, she sure knows how to use her noodle.