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.
(..) Yeah, science. Okay. - What's this? - That is a shopping list. Getting some of those items may be challenging. One autotransformer... ... litres anhydrous methenamine. - Two M and M tube furnaces. - It's mm, millimetre. One millimetre would be fine, (..)2008 TvShow: Breaking Bad Title: A No-Rough-Stuff-Type Deal Season: 1 Episode 7