"It's time to see how clearly you can think," the teacher said to his class. "Now, listen carefully, and think about what I'm saying. I'm thinking of a person who has the same mother and father as I have. But this person is not my brother and not my sister. Who is it?"

The kids in the class furrowed their brows, scratched their heads, and otherwise showed how hard they were thinking. But no one came up with the right answer.

When everyone in the class had given up, the teacher announced, "The person is me."

Little Jeffrey beamed at learning the answer. "That's a good one," he said to himself. "I'll have to try that on Mom and Dad."

At dinner that night, little Jeffrey repeated the riddle to his parents. "I'm thinking of a person who has the same mother and father as I have," he said. "But this person isn't my brother and isn't my sister. Who is it?"

His parents furrowed their brows, scratched their heads, and otherwise pretended that they were thinking hard. Then they both said, "I give up. Who is it?"

"It's my teacher!" Jeffrey said.