The 4th grade teacher noticed that Derick had been daydreaming, so she decided to get his attention.
“Derick,” she said, “if the world is 24,901 miles around and eggs are two dollars a dozen, how old am I?”
“Thirty-four,” Derick answered unhesitatingly.
The astonished teacher replied, “Pretty good. That’s not far from my actual age. So tell me, how did you guess?”
“It was simple,” Derick said, “My older sister is seventeen and she’s only half-crazy.”