A: You did all the housework yourself.
B: Well, not quite all of it. I still have some ironing to do.
A: So, that's what you're doing, ironing. Um, what's ironing?
B: Oh, let me show you. This is the iron. Oh, it's hot. I press it down on the shirt.


Which is grammatically correct:

1- that's what you're doing, ironing < with comma

2- that's what you're doing ironing < without comma

Thank you


Use the comma. The sentence is clearer that way. In speech there's a short pause where the comma is.



Thank you, Cool Breeze