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?
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.
What moves forwards and backwards but has no legs, it cannot talk, it cannot walk, it doesn't slither or crawl across the floor either. It comes in different shapes, sizes and colours. What is it?