Simon badly wanted to say: I was here first. But he didn't. It was remarkable, a dispassionate little corner of his mind observed, how someone with his face could bring out the childishness in him. Don't copy me, and all that. He'd never been like that with his own sister, and had pitied children whose sibling relationships were less idyllic than his own.

Maybe it was like catching chicken pox late in life. Worse, the older you caught it.

"If there's an explanation," he said, "I'm not the one who ought to be giving it. Do you know where you are?"