The slope was 'challenging' for even the most experienced climbers.
A number of doctors are 'challenging' the study's claims.
It's a 'challenging' book that requires careful reading.
(..) No, I'm afraid I'm gonna have to challenge that. No, you don't have to challenge that. That's a word. - That's a definite word. - I am challenging. "Quone." To "quone" something. I'm not playing with you anymore. "Quone's" not a word. No good. Sorry. There it is. Get it off. (..)1990 TvShow: Seinfeld Title: The Stakeout Season: 1 Episode 2