Kindergarten Assignment - the Missing Period

The 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 for the little kids to give their reports, the teacher was calling on them one at a time.

The teacher was reluctant to call upon little Johnnie, knowing that he sometimes could be a bit crude. But eventually his turn came. Little Johnnie walked up to the front of the class, and with a piece of chalk, made a small white dot on the blackboard, then sat back down.

Well, the teacher couldn't figure out what Johnnie had in mind for his report on something exciting, so she asked him just what that was.
"It's a period", Johnnie explained.
"Well I can see that," she said, "but what is so exciting about a period?"

"Darned if I know," said Johnnie, "but this morning my sister said she missed one. Then Daddy had a heart attack and Mommy fainted."