Effect: Grab a glass, any glass... a pint glass, a cocktail glass... even a shot glass. Ask everyone at the table to take out a dollar, and then ask them which is longer: the circumference around the rim of the glass, or the height of the glass, from the table all the way up to the rim. Make 2 piles with their dollars: one for people who say the circumference is longer, and one for people who say height. You'll find the majority of people will say "circumference" at first (make sure to withhold your bet until the end).
Next, place a pack of cigarettes (or a deck of cards) under the glass, and repeat the question. Now that it's obviously taller, a few of the players will now defect to the "height" side. Move their dollars over to the height pile. Then add another pack of cigarettes under the glass, and repeat. Keep going round by round until you're the only person left who will bet on circumference. Place your lone circumference bet, and then measure: unfold a paper napkin, wrap it around the rim of the glass, and unwrap it to show that even with the extra added height, the circumference is still greater!Secret: People are just really, really bad at estimating circumferences. A pint glass is almost a foot around, but you'd never think it by looking at one. Before the trick, get an estimate of the circumference by wrapping your fingers around the rim of the glass and unwrapping them next to it. Just make sure when you're adding supports underneath the glass, you stop just before the height matches the circumference.
Tip: It's important to get everyone's money on the table for this one. If they have a buck riding on it, they'll go with their true (wrong) estimates. If there's nothing at stake, I find that many people will keep saying "circumference" just to be contrary.