What makes a task challenging?

A challenging task takes us to the very limits of our ability and pushes us to go just a bit further.  That's why not everyone finds the same tasks challenging. 

For example, a group of lower-level students tasked with using adjectives to describe something without being allowed to repeat adjectives used by anyone else in the group will find this challenging because they don't know that many adjectives, whereas a higher level group will find the task easy because they have more language at their disposal.