The teacher noticed that Mike had been daydreaming for a long time. She decided to get his attention.
"Mike," she said. "If the world is 25,000 miles around and eggs are three dollars a dozen, how old am I?"
"Thirty-four," Mike answered without hesitation.
The teacher replied, "Well, that's not far from the truth. Tell me ... how did you guess?"
"Oh, there's nothing to it," Mike said. "My big sister is seventeen and she's only half-crazy."