sharp as a tack
Also, sharp as a razor. Mentally acute. For example, She's very witty—she's sharp as a tack. These similes are also used literally to mean “having a keen cutting edge” and have largely replaced the earlier sharp as a needle or thorn. The first dates from about 1900, the variant from the mid-1800s.
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Example sentences from the Web for sharp as a tack
ROME — What does it take for a Hollywood A-lister to get a private audience with Pope Francis?Pope Francis Has the Pleasure of Meeting Angelina Jolie for a Few Seconds|Barbie Latza Nadeau|January 8, 2015|DAILY BEAST
Pulling oil from the tar sands is costly, even more so when you tack transportation costs on top.
Yes, Byrd—dead four-and-a-half years now—was a Kleagle in the Ku Klux Klan.Steve Scalise and the Right’s Ridiculous Racial Blame Game|Michael Tomasky|January 2, 2015|DAILY BEAST
By the time it concluded with a sing-a-long of “XO,” Beyoncé had done the rare thing.Bow Down, Bitches: How Beyoncé Turned an Elevator Brawl Into a Perfect Year|Kevin Fallon|December 31, 2014|DAILY BEAST
Having graduated Juilliard last spring, Alex Sharp is too young to have given the performance of a lifetime.Hedwig, Hugh & Michael Cera: 12 Powerhouse Theater Performances of 2014|Janice Kaplan|December 31, 2014|DAILY BEAST
Now-a-days it is the bankrupt who flouts, and his too confiding creditors who are jeered and laughed at.Glances at Europe|Horace Greeley
A sob rose in her throat, and broke from her lips transformed into a trembling, sharp, glad cry.The Bondboy|George W. (George Washington) Ogden
He felt himself the meanest, vilest thing a-crawl upon this sinful earth, and she—dear God!St. Martin's Summer|Rafael Sabatini
All that scientific bric-a-brac in the cupboard had far better be thrown away.The Salvaging Of Civilisation|H. G. (Herbert George) Wells
It was one of those long moments that makes a fellow draw his breath sharp when he thinks about it afterward.Raw Gold|Bertrand W. Sinclair