This is a fun prank you can pull on most people. Tell someone that you, or your friend or significant other, is one-third {any nationality or religion}. Most people will believe you right off-the-bat, as if you said one-half or one-quarter. I did this the other night, and the woman I was talking to started talking about how she too had Polish heritage.
The problem is, you can’t be one-third anything. Since parents only come in pairs, you can only be a fraction of a power-of-two. Five-sixteenths comes close to one-third, so you can approach 1/3, but never get it exactly.
The way I envision getting free beer out of the deal is starting out by claiming that your friend says he is 1/3 Polish, but you can prove that it’s not so with only a pen & blank sheet of paper (no birth certificate or other documentation). If your mark doesn’t believe you can prove it, you can bet just about anything you’d like. After a few minutes, you should be able to convince them. Two is a prime number, so no power-of-two can be divisible by three. Make sure your mark isn’t too drunk to understand simple math.
I completely got fooled by this one, even though my brain is fully capable of doing the proof. But then, for years I thought Obi-Wan Kenobi said he heard a million oysters cry out and then they were suddenly silenced. (It was voices, not oysters.)
Have fun!