Squigly Squigly's Brain Teasers

Question:

How many tracks does a monorail have?

Answer:

One.

Try another Brain Teaser:

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?