E=MC^IRISH

Ingredients: 1 empty beer bottle, and a small piece of napkin.
 
Objective: Get a free drink.
 
Situation: Hold an empty beer bottle horizontally. (Make sure inside of neck is dry.) Tear off a small piece of napkin, wad it up into a small ball. Place the napkin wad just inside the lip of the bottle. Hold the bottle in front of someone's face, and bet them a drink that they can not blow the napkin into the bottle.
 
Solution: Bernoulli principle states that moving air has lower pressure than still air. Therefore, when your friend blows, the still air in the bottle will push the napkin out of the bottle, right back at the face of the blower.
 
Note: The harder they blow, the faster the napkin will come out. If you get a really gullible person, you can have them try again for a chance at another drink. You should be able to milk them for at least 3 or 4.