A sure fire laugh getter! An assistant from the audience is called upon to help you. Before they touch your props, they are asked to wipe their hands on a napkin given to them. 

The napkin is casually shown to be clean on both sides. After the assistant wipes their hands the napkin is covered with black hand prints! 

This is guaranteed to get you a big laugh. 

Trick napkin may be used over and over again.