use one's head

[use one's head] or {slang} [use one's bean] or {slang} [use one'snoodle] or {slang} use [one's noggin] {v. phr.} To use your brain ormind; think; have common sense. - Often used as a command.

If youused your bean you wouldn't be in trouble now.

Never point a gunat anybody, John. Use your head!