General:
* The new project was challenging, but ultimately rewarding.
* Learning a new language can be challenging, but it's also incredibly fulfilling.
Specific examples:
* The mountain climb was challenging due to the steep terrain and unpredictable weather.
* This math problem is challenging even for the most advanced students.
* The doctor explained that her recovery would be challenging but not impossible.
Figurative:
* She faced a challenging decision about her future.
* His new job was challenging in a good way, forcing him to step outside his comfort zone.
As a verb:
* The instructor challenged us to think critically about the topic.
* The opposing team challenged the referee's call.
The word "challenging" can be used in many ways, depending on the context.