The husband says, "I'll go get you one."

"I better write you a note. I know how forgetful you are." "No, no, I can remember that, simple enough, you want an ice cream sundae."

"And I love peanuts on top, you know that."

"Of course. I won't forget."

"Are you sure? I'd better write you a note, so you remember to order the peanuts on top."

"No, no, I'll remember that."

So he goes off for a while and comes back with a Big Mac and fries. His wife says, "I knew you'd forget the ketchup!"