A boy goes to the drug store with his dad and sees the condom display. 

Boy: "Dad, why do they do packs of one condom?" 

Dad: "Those are for the high-schoolers for Friday nights." 

Boy: "So, why do they make packs of three?" 

Dad: "For the college guys for Friday, Saturday and Sunday nights." 

Boy: "Then why do they make packs of 12?" 

Dad: "Those are for married couples -- you know, January, February, March.