This is legit and people don’t realize it.

“hey what are you doing?”
“nothing” 
“oh great! so you are avaliab-”
“no you don’t understand. I’m doing nothing.” 

I have learned the following:

“Hey, what are you doing?”
“Why? What’s up?”

Then you have the entire space of their answer to come to terms with potentially doing something, or come up with a better thing to say than ‘nothing’.

Alternately, I find that answering with ‘relaxing’, totally conveys the right mood and people then generally reply with, “ohhhh I know that feeling, no problem, go relax!”