adjective
describes a noun or pronoun, in this case, 'challenging' describes something that is difficult and requires effort or skill
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The project presented many challenging obstacles that required innovative solutions.
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I find sudoku puzzles to be quite challenging, but also very rewarding.
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Dude, that skateboarding trick is so challenging, you gotta try it!
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Overcoming the loss of a loved one can be one of the most challenging mountains to climb in life.