Knock-knock

I went to the school today to have lunch with the oldest. 14 first graders and I sat at one table. One child's mother puts riddles in his lunchbox every day. Today's riddle was

What type of dress do you have at home but can't ever wear?
Answer: Your Address.

I decided to teach the children my favorite knock-knock joke from my childhood.

Knock-knock.
Who's there?
Interrupting Cow.
Interrup-
MOO!

Which is a wonderful joke, once one masters the art of

timing
noun
1. The regulation of occurrence, pace, or coordination to achieve a desired effect, as in music, the theater, athletics, or mechanics.
2. The synchronization of the sparking of the plugs with the movement of the pistons in an internal-combustion engine.
"timing." The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. Answers.com 23 Jan. 2006. http://www.answers.com/topic/timing

And apparently quite hilarious to six and seven year olds.