Little Johnny is always being teased by the other neighborhood boys for being stupid.
Their favorite joke is to offer Johnny his choice between a nickel and a dime Little Johnny always takes the nickel.
One day, after Johnny takes the nickel, a neighbor takes him aside and says, "Johnny, those boys are making fun of you. Don't you know that a dime is worth more than a nickel, even though the nickel's bigger?"
Johnny grins and says, "Well, if I took the dime, they'd stop doing it, and so far I've made $20!"