The Bet:

Introduce a tall glass, like a Zombie glass, and wager that the distance around the rim of the glass is greater than the height of the glass.

Most people would bet against you. To prove what you say is true use a piece of paper, like a napkin or dollar bill, to measure the height of the glass. Have someone mark the paper to record the height of the glass. Allow someone in the audience to wrap the same piece of paper around the rim of the glass and most will be astonished that the circumference is usually much greater than the height of the glass.

You’ll want to try various types of glasses but a Zombie glass like the one pictured here works best.
If you know the circumference of the mouth of the glass prior to betting, then you would also know how much stuff you can stack underneath the glass and have it still be shorter in height. I’ll put coasters and my cell phone and other things underneath the glass to apparently sweeten the odds for my victim.

Proclaim, “I never make mistakes. I thought I did once, but I was mistaken.”