So what? This is very situational

For example, if you boss ask you to take a task, and you want to say NO.

Not just simply say No.

Instead, you would recommend that your college A take this task. If A is a junior, you would say that this is a good opportunity for A to learn and I would help A. If A has more experience, you would say that this is an importnat task, and A is more qualified for this task.

Happy Now?