This info has been circulating on the internet since 2002 but this is the first time I've seen. Kinda cool and creepy at the same time. I guess some people has too much time on their hands to come up with stuff like this.
1. Fold a $20 bill in half so that you see the top half of the reverse side.
2. Fold the left half away from you as shown.
3. Fold the right half so that the burning Pentagon is revealed.
4. Flip the bill over to see the World Trade Center.