What are the Secrets on the five dollar bill?

The secret is that if you put it in the light and on at it you will see a face on the right or left side depending on which side you look at it. Also you cannot see the face unless you put it in the light.

CorrectionThe face is called a watermark. It's a security feature that's used on all modern US bills except $1 and $2. There's nothing secret about it whatsoever. In fact, the Treasury wants people to be aware of it as a way to reduce counterfeiting.