From Father To Son
A boy goes to the drug store with his dad and sees the condom display. The boy asks: "Dad, why do they make packs of just one condom?" Dad says: "Those are for the high-schoolers for Friday nights."
Then the boy asls: "So, why do they make packs of three?" And dad says: "Those are for the college guys for Friday, Saturday and Sunday nights."
Finally the boy asks: "Then why do they make packs of 12?" And dad tells him: "Those are for married couples -- you know, January, February, March..."