You cannot tell the mass, because what you are doing now is appealing to the notion of mass as something that's related to the pull of the Earth on the object.

你无法测出质量,因为你现在所做的是把质量,用地球对物体的引力给替代了

耶鲁公开课 - 基础物理课程节选

Use this pattern when you wish to express what you are doing right now.

当你想表达目前你正在做什么的时候，你就可以用这个句型。

I'm ~ing 课堂 - SpeakingMax英语口语达人

But as you get older and you have a lot of spare time, you think about what you are doing, which is something I have the luxury of doing right now, and I realized this is more tricky.

但随着你们年龄的增长,有更多空闲时间之后,你就会思考你做的事情,我现在就有很多闲暇思考这些问题,我意识到这东西其实很精妙

耶鲁公开课 - 基础物理课程节选

What kind of internships are you doing right now?

你现在在做什么样的实习？

正在实习 - SpeakingMax英语口语达人