A Small White Dot

Here is an oldie that Irene sent me.
Maybe there is one person left who hasn't heard it.

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, Johnny, 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," Johnny replied.
"I can see that," said the teacher, "but what is so 'exciting' about a period?"
"Darned if I know," Johnny said, "but this morning my sister Susie said she was missing one. Mommy fainted, Daddy had a heart attack, and the boy nextdoor joined the Navy."