[use one's head] or {slang} [use one's bean] or {slang} [use one's noodle] or {slang} use [one's noggin] {v. phr.}
To use your brain or mind; think; have common sense. — Often used as a command.
* /If you used your bean you wouldn't be in trouble now./* /Never point a gun at anybody, John. Use your head!/