A little boy is walking around a supermarket with his Dad, when he sees some condoms on a shelf.
"What are they, Dad?" the boy asks.
"Well, you use one when things start to get interesting between a man and a women" his Dad replies.
"Why would you need a pack of three, then Dad?" he asks.
"Well, if you get lucky then there's one for Friday night, one for Saturday night and one for Sunday night," replies his Dad.
The boy continues, "Well, why would you need a pack of six, Dad?"
His Dad replies "That's for if you get really lucky. One for every night of the week; Monday, Tuesday, Wednesday, and so on through to Saturday."
By this point the boy is getting intrigued. "So Dad, what is a pack of 12 used for?"
"Now," says the boy's Dad, "that's for when you're married. One for January, one for February...."