Show a spall black tube, and ask someone a banknote. Roll it, and put it in the tube. Close it and shake it. Open teh tube and cough up the banknote to its owner. But when you return the tube, there isn't any banknote, But some water !
This trick is very easy to realize : it is totally automatic.
The tube can be examinate.
You can transform lots of small items as rice, salt or whatever you can find.