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."