convinced to convince

 “What’s your shoe size?”
    This was the Professor’s first question, once I had announced myself as the new housekeeper. No bow, no greeting. If there is one ironclad rule in my profession, it’s that you always give the employer what he wants; and so I told him.
“Twenty-four centimeters.”
“There’s a sturdy number,” he said. “It’s the factorial of four.” He folded his arms, closed his eyes, and was silent for a moment.
 “What’s a ‘factorial’?” I asked at last. I felt I should try to found out a bit more, since it seemed to be connected to his interest in my shoe size.”
 “The product of all the natural numbers from one to four is twenty-four,” he said, without opening his eyes. “What’s your telephone number?”
 He nodded, as if deeply impressed. “That’s the total number of primes between one and one hundred million.”
 It wasn’t immediately clear to me why my phone number was so interesting, but his enthusiasm seemed genuine. And he wasn’t showing off; he struck me as straightforward and modest. It nearly convinced me that there was something special about my phone number, and that I was somehow special for having it."

The housekeeper and the Professor - Yoko Ogawa