have a mind as sharp as a steel trap
To be able to understand or grasp information quickly. David has a mind as sharp as a steel trap, so if you give him the instructions, he'll follow them perfectly.
have a mind as sharp as a steel trap
Fig. to be very intelligent. She's a smart kid. Has a mind as sharp as a steel trap. They say the professor has a mind as sharp as a steel trap, but then why can't he figure out which bus to take in the morning?