How many times can you subtr...

A kindergarten class had a homework assignment to find out about 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 walked up to the front of the class made a small white dot on the blackboard and sat back down. Puzzled, the teacher asked him just what it was.
"It's a period,'' said the little boy.
"Well, I can see that,'' she said, ''but what is so exciting about a period?''
''Damned if I know,'' said the little boy, ''but this morning my sister was missing one, Daddy had a heart attack, Mommy fainted, and the man next door shot himself."