Synonyms: challenging, ambitious
Definition: requiring full use of your abilities or resources
Usage: ambitious schedule; performed the most challenging task without a mistake
Similar words: difficult, hard
Definition: not easy; requiring great physical or mental effort to accomplish or comprehend or endure
Usage: a difficult task; nesting places on the cliffs are difficult of access; difficult times; why is it so hard for you to keep a secret?
Synonyms: challenging, intriguing
Definition: disturbingly provocative
Usage: an intriguing smile
Similar words: provocative
Definition: serving or tending to provoke, excite, or stimulate; stimulating discussion or exciting controversy
Usage: a provocative remark; a provocative smile; provocative Irish tunes which...compel the hearers to dance- Anthony Trollope
Synonyms: thought-provoking, challenging
Definition: stimulating interest or thought
Usage: a challenging hypothesis; a thought-provoking book
Similar words: stimulating
Definition: rousing or quickening activity or the senses
Usage: a stimulating discussion