You show a deck of cards for the audience to choose freely (no force) and then the selected cards can be visually sticky to a pen or pencil.
As well as cards, you can borrow bills from the spectators and at the end of the show you can give pencils/pens and bills to the spectators to do so. Well... of course the spectator won't be able to do it, because it's MAGNETIC MAGIC!
If the spectator thinks it's magnetic, then it may stick iron or the like on a pen or pencil but that won't work. This is a very killer effect because before and after the show can be directly checked by the audience without having to distract them.
Note:
Before applying the spectator can feel the surface of the pencil