What is this green paper all about?

I had this twenty folded up in a plastic bag with just the face of Andrew Jackson Showing,  Viola said that she wanted to see the photo in the bag.  Then I realized that she has possibly never held real money.  She found it amazing, then I tried to explain how debit cards work since that is how we spend our money.  It was a very financially educational afternoon.

 Then she tried to carry it off to her room.  End of money education time.  Wait, Education continued, "that's a lot of money."