be as sharp as a needle
be as sharp as a tack

He was as sharp as a needle, and in half an hour was quite able to pick up twenty or more new words.

It was made of a hard, glossy wood, and its point was as sharp as a needle.

Old Miss Marple has always been as sharp as a needle, and I'd say she still is.'

He looked placid and easygoing, and if truth be told, sleepy, and yet Phyllida was aware that underneath all that he was as sharp as a needle, ready to fire awkward questions at her and make remarks she couldn't understand.

He is as sharp as a tack.

Nope, Bud was as sharp as a tack."

Dan comments 'Paceley was as sharp as a tack and super dependable.