An extremely visual rubber band through pen with a borrowed pen!
The gimmick is very easy to make and can be carried around with you at all times.
You can borrow a sharpie or use your own as you make a rubber band pass trough the pen very visually!
The trick then instantly resets.
There can sometimes be some lighting issues, but in most one on one situations, this can be handled very easily.
The method has been said to be more amazing then the effect itself!