"What are you doing?" is the English way to ask someone what they are doing. It comes from the English verb "to do," which is used to describe actions and activities. Here are some example sentences in English and their translations in Chinese:

"What are you doing?" 你在干什么？

"I'm just reading a book." 我只是在看书。

"I'm doing my homework." 我在做作业。

The verb "to do" is a very common and versatile verb in English. It can be used to describe a wide range of activities, from simple tasks like reading or writing, to more complex activities like working or studying. The verb "to do" is often used in questions like "What are you doing?" to ask someone about their current activity or state of being.