Nuts and Bolts
A pure mathematician admits to not even knowing the 'nuts and bolts' of what nuts and bolts are.
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Cartoon by Ben Orlin, Math with Bad Drawings. https://cartoons.mathwithbaddrawings.com/2018-06-28-nuts-and-bolts/ (CC BY-NC 4.0)Transcript
A: "I know you're a 'pure' mathematician, but you still know some nuts and bolts of engineering, right?"
B: "I don't even know the nuts and bolts of what 'nuts and bolts' are."