Nine year old's knees...

....should always look like this.

There's a story about a little child who had just started school. At the end of his first day the mother came into the classroom to see the teacher - she was not happy! "Take a look at the state of my son's uniform!" she said. The baffled teacher looked him up and down. He looked exactly as he did at the start of the day: clean shoes, clean shirt, clean trousers...in fact, not even a hair was out of place on his clean head.
"What's the matter?" said the teacher, "He looks perfect."
"Exactly!" said the mother, "How can he possibly have had fun if he's coming back looking like that?"

Dirt can be washed off quickly, enjoyment and fun remain in the memory for a lot longer.