One Little Dot

One Little Dot...
The class was given a homework assignment to find out something exciting and relate it to the class the next day.
When the time came to present what they'd found, the first little boy the teacher called on walked up to the front of the class. He picked up a piece of chalk, made a small white dot on the blackboard and sat back down.
Puzzled, the teacher asked him what it was.
"It's a 'period'," he replied.
"I can see that," said the teacher, "but what is so exciting about a 'period'?"
"Darned if I know," said the boy, "but yesterday my sister announced she was missing one. Mom fainted, Dad had a heart attack...and the boy next door joined the Navy."