Bern tells Sandy, "This isn't the $20 bill you left on the table. It is mine - I was keeping it between pages 15 and 16 of my textbook."
Sandy retorts, "You are lying and I can prove it." How does she know?
Pages 15 and 16 are on the same sheet of paper in the textbook (just like pages 1 and 2 are) and therefore the bill couldn't be between them.