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.
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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.