A father and his son go into the grocery store when they happen upon the condom aisle. The son asks his father why there are so many different boxes of condoms. The father replies, "You see that 3 pack? That's for when you're in high school. You have 2 for Friday night and 1 for Saturday night."
The son then asks his father, "What's the 6 pack for?" The father replies, "That's for when you're in college. You have 2 for Friday night, 2 for Saturday night, and 2 for Sunday morning."
Then the son sees the 12 packs, with his eyes wide open, he asks, "What about the 12 pack?" The father replies, "That's for when you're married. You have one for January, one for February, one for March..."