“Hi, Daddy,” I said as I came into our apartment. “Am I late?”

The living room was in a real mess. Books and papers were all in piles on the floor and the furniture was all shoved to one side. Our home ordinarily had a lived-in look, but this was far worse than usual.

Daddy was sitting in one of the chairs, sorting books. Daddy is Miles Havero. He is a small man just into middle age with a face that is hard to read, and a very sharp mind. He is mainly a mathematician, though he sits on the Ship’s Council and has for years. He and I had lived in this apartment since I left the dormitory when I was nine.

He gave me an inquiring look. “What happened to you?”

“I didn’t mean to be late,” I said.

“I didn’t mean that,” he said. “I’m talking about your clothes.”

I looked down. I had on a white shirt and yellow shorts. Across the front of both were streaks of dust and grime.

The Ship is a place where it is almost impossible to get dirty. The ground in the quad yards isn’t real dirtand-grass, for one thing. It’s a cellulose product set in a milled fiber and plastic base — when a square gets worn they rip it out and put in a new one, just like in your living room floor. Tbe only place there is dirt in any quantity is the Third Level, where there isn’t anything else but. A certain amount of dirt does get carried out of the Third Level and spread and tracked around the Ship. Eventually it gets sucked into the collecting chutes and blown down to Engineers on the First Level, where it is used to feed the Convertors to produce heat, light and power inside the Ship. But you can see that ordinarily there isn’t much opportunity to get filthy.

I once asked Daddy why they didn’t work out a system to keep the dirt at its only source — the Third Level — instead of going to the trouble of cleaning the Ship after it gets dirty. It wouldn’t be hard to do.

He said, “You know what the Ship was built for, don’t you?”