Let me give you an example. When we were young, we often volunteered to go out to work at school. As students, do we have the obligation to work? This is a truth. It is not impossible to do something within your power without affecting your normal work.
Besides, if you have a job, the boss will never let you move bricks. Maybe you have time to move bricks now. On the other hand, it is not unreasonable for the boss to pay you. Maybe if you move bricks, you should use them to build fences, which will save labor and let you know how to maintain public property.
I hope I can help you!