have a mind like a steel trap
To be able to understand or grasp information quickly. David has a mind like a steel trap, so if you give him the instructions, he'll follow them perfectly.
mind like a steel trap, have a
Be very quick to understand something, as in Aunt Ida may be old, but she still has a mind like a steel trap. This simile likens the snapping shut of an animal trap to a quick mental grasp.