Usually, it goes something like this...
"Milo, what are you doing?" (Called down the hallway, down the block, across the playground, on the phone...)
"Just playing, Mama."
"What are you really doing, Milo?"
"I'm busy working." (Building castles, typing on an old keyboard, reading, dumping toys on the floor...)
"Milo... Milo!"
"Just playing."

Here he is, really just playing, with Ahmad on the swings in matching pants...

And with Gabriel and The Water Balloon That Went Ploop.