sharp as a tack

(as) sharp as a tack (quick to understand, able to think quickly) — понятливый, долго объяснять не приходится; светлая голова; за словом в карман не полезет

Example: Amanda scored 100% on her math test. She's as sharp as a tack.

see also
[quick on the uptake]
[bright as a button]
[think on one's feet]