This bloke goes into a chemist to buy some condoms. The assistant asks "Which do you want, a twin pack, six pack or twelve pack?" The bloke replies "I don't know. What's the difference?"
The assistant responds "Well, teenagers usually buy the twin pack, one for Friday and one for Saturday. College students will buy the six pack, one for Monday, one for Tuesday, one for Wednesday, one for Thursday, one for Friday and one for Saturday.
And then there are your married couples who buy the twelve pack. One for January, one for February, one for March."