You bet the drunk a beer that you can beat him in a simple game.  By taking turns (you going first) stick a nickel or bottle cap on your head and try knocking it off without using your hands.  Shake your head; tap in on the bar; do whatever.  Sounds simple enough.

The trick:

By pushing a nickel hard against your forehead it will stick.  Do this in front of the drunk.  Once you stick it to your own forehead, slightly tap your head on the bar and knock it of.  By now the drunk thinks this is easy.  Once you knock it off of your own head, tell the drunk it's his turn and you bet him a beer that he can’t do it.  Ask him to lean over the bar while you push the nickel against his head.  Tell him you have to push hard to get it to stick.  After about ten seconds of pushing, pull the nickel off of his head, being as slight as possible, without him knowing.  By pushing on the nickel as hard as you did, the drunk will think the nickel is still on his head.  With a little encouragement, get the drunk to knock the nickel off without his hands.  It's the funniest thing! They will repeatedly smack their head against the bar, shake it around, and make funny faces trying to get the nickel off, while you, and the rest of the bar, laugh your ass off.  Note: Be careful when doing this trick at a bar with a mirror behind it. It will spoil the trick.

I hope this makes sense.

Brett.

Spoon's_forks+knives sent in this version;

Face Embarrassment: Take a bottle cap and press it against your forehead until it sticks. Then skrunch up your face a couple times and it will pop off. Now press it up on the victims forehead (but secretly pull it back off.) Tell him that only you can get it off. (make sure he doesn't use anything to get it off.) He will think it is still on his forehead because of the feeling of the mark. he will mess his face up and completely embarrass himself. When your satisfied with the prank show him the bottle cap in your hand. DON'T BLAME ME IF YOU GET A BLOODY NOSE!!!!