Jack tells Jill, "This isn't the $5 bill you left on the the table. I found it between pages 15 and 16 of Harry Potter."

Jill retorts, "You're lying and I can prove it." How did Jill know?

Answer:

Harry Potter, like all other books, has odd-numbered pages on the right. Therefore, pages 15 and 16 are the front and back of a single page, and nothing could have been found between them.

Try another Brain Teaser:

Which is measured from the circle's centre: the radius or the circumference?