Two intelligent, honest students are sitting together at lunch one day when their math teacher hands them each a card. "Your cards each have an integer on them," the teacher tells them. "The product of the two numbers is either 12, 15 or 18. The first to correctly guess the number on the other's card wins."
The first student looks at her card and says, "I don't know what your number is."
The second student looks at her card and says, "I don't know what your number is, either."
The first student then says, "Now I know your number."