Alright, here's another old one for the oldies and a new one for the youngsters:
Boy walks into chemist to buy condoms for the first time in his life.
Boy: "I'd like a packet of condoms please"
Chemist:"Certainly sir, what number pack"
Boy:"What number?"
Chemist:"You can have a 3 pack, a 6 pack or a 12 pack"
Boy:"Well, what's the difference?"
Chemist:"Well, a 3 pack is for when you're courting - once on friday, once on saturday and once on sunday. A 6 pack is for when you're engaged - twice on friday, twice on saturday and twice on sunday. And a 12 pack is for when you're married - once in january, once in february, once in march......"