1. 
   challenging
   [adj] - stimulating interest or thought 2. [adj] - disturbingly provocative
   Found op http://www.webdictionary.co.uk/definition.php?query=challenging
   
   
2. 
   challenging
   intriguing adjective disturbingly provocative; `an intriguing smile`
   Found op http://wordnetweb.princeton.edu/perl/webwn?s=challenging
   
   
3. 
   challenging
   thought-provoking adjective stimulating interest or thought; `a challenging hypothesis`; `a thought-provoking book`
   Found op http://wordnetweb.princeton.edu/perl/webwn?s=challenging
   
   
4. 
   Challenging
   • (p. pr. & vb. n.) of Challenge
   Found op http://thinkexist.com/dictionary/meaning/challenging/
   
   
5. 
   challenging
   1) Ambitious 2) Confrontational 3) Difficult 4) Hard 5) Insulting 6) Offensive 7) Provoking 8) Stimulating
   Found op http://www.mijnwoordenboek.nl/EN/crossword-dictionary/challenging/1
   
   

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