have (one's) thinking cap on
To be using, or be ready to use, the utmost of one's intellectual abilities; to be able or ready to think very carefully and seriously (about something), often to solve a problem. I'm really excited for the pub quiz tonight! Does everyone have their thinking caps on? OK fellas, if we all have 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.