Little white dot
 

A kindergarten class had 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 and with a piece of chalk, made a small white dot on the blackboard and then 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," he said, "but this morning my sister was missing one.
Mommy fainted, Daddy had a heart attack, and the boy next door joined the Navy."