Did he look like he has just come out from a coal mine? Or has just completed his art painting session, with his knees? The answer is none of both.
He had just come back from school. He got filthier by the day. He has loads of reasons when questioned for coming back in such filth.
'We sat on the floor' which means dirty pants.
'Teacher asked us to walk on our knees during PE' which means dirt-stained knees.
He couldn't explain 'black' palms except that he must have crawled on the floor, or worse still, lied on the floor as his t-shirt was not spared from the dirt.