sharp as a tack

Example: “At ninety-four, my great-aunt is still sharp as a tack.”

Origin: Turns on a pun that English has carried for over a thousand years: "sharp" has meant both physically pointed and mentally quick since Old English, so the comparison does not have to build a metaphor, only exploit one already sitting in the word. The tack itself is doing less work than the sound is. Earlier versions of the comparison used other pointed objects — sharp as a needle, sharp as a razor — and a tack is not meaningfully sharper than either; what it has is the matching consonant, which makes the phrase easier to say and to remember. The tack version is American and twentieth-century, and it has largely displaced the older variants for no reason other than that it sounds better.