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.
(..) Don't turn your back on me, Scar. On, no, Mufasa. Perhaps you shouldn't turn your back on me. Is that a challenge? Temper, temper. I wouldn't dream of challenging you. Pity! Why not? Well, as far as brains go, I got the lion's share. But, when it comes to brute strength... (..)1994 The Lion King