put thinking cap on
put (one's) thinking cap on
To begin thinking very hard about something, especially a solution to a problem. If we all put our thinking caps on, I'm sure we can come up with a way to earn enough money to go to the baseball game this weekend. A: "Well, boss, what do you think we should do?" B: "Just let me put my thinking cap on for a minute."
put one's thinking cap on
Fig. to start thinking in a serious manner. (Usually used with children.) It's time to put our thinking caps on, children. All right now, let's put on our thinking caps and do some arithmetic.