Hi 哈里, When we are focusing mentally for a period of time on a task, we say that we are "working on" something. For example: We are working on a science project. / We are working on a flying prototype. When we ask someone "what they are doing", it could be on something which they are doing at the moment. Example: Question: What are you doing? Answer: I am.waiting for the bus (which is temporal and does not need a great deal of mental focus). Hope this helps. Cheers, Lance

"What are you doing?" is more general. It can refer to a variety of things, including work, play, relaxation and etc. "What are you working on?" implies that the person is working on something.