A woman with six children in tow approached the train fare counter and
asked for five tickets. "One for me", she said, "then two full-fare for
John and Jack who are 16; two half-fare for Millie and Mollie who are
six;
and none at all for Tim and Tom who are only four."

The ticket agent stared at the brood of twins. "Do you mean to tell me,
madam," he said at last, "that you get two children every time?"

"Of course not," the woman replied. "Usually, we don't get any at all."