Hello Justin. You made a very good question. Let me explain.
In Modern Greek "τι κάνεις" means: a) What are you doing b) How are you. So it's an ambiguate question.
That's why among the native Greek speakers it's very common that when you are asked this question you answer both:
Α: Τι κάνεις;(=What are you doing+How are you?)
Β: Καλά είμαι, διαβάζω.(=I'm fine! I'm studying right now)
Sometimes even when we want to ask the other person only "What are you doing" due to the ambiguity of the phrase the other person thinks that we are asking only "how are you":
A: Τι κάνεις(=What are you doing?)
B: Καλά είμαι(I'm fine).
Then what do we do in this occasion to solve the ambiguity? We need specify:
A: Τι κάνεις(=What are you doing?)
B: Καλά είμαι(I'm fine).
A: Όχι, εννοώ τι κάνεις στην πραγματικότητα!(=Νο, I am asking literally what are you doing!)
Β: Αααα! Διαβάζω τώρα!(=ohhh! I am studying right now!)
Kind Regards,
Stergios Efes