Next time you check into a hotel, here's a little prank you can try on the desk clerk.
When he asks you for your credit card as insurance, ask him to feel the serial number on the card with his forefinger and then without warning him, just wipe the numbers off by rubbing your finger over them. If he doesn't fall out of his chair, it is because, he'll be to busy gawking in disbelief.
-Easy to master and even easier to perform
-Examinable, before and after the effect
-Don't leave home without it!