"Pardon me for asking," said Nasruddin, "but doesn't it usually take nine months for babies to be born?"
"That's right," she replied.
"But then how could you have had this child so soon?" he asked.
"It's simple arithmetic," she answered. "How long have you been married to me?"
"And how long have I been married to you?"
"And how long has the baby been growing inside me?"
"There you go!" she concluded triumphantly. "Nine months, just as it should be."
Inspired by: Mulla's Donkey and Other Friends by Mehdi Nakosteen
Notes: This story is on p. 47 of the book.