Put a quarter on the table, with George Washington's head right side up. Hold the quarter down with your finger. 

Now put another quarter flat on the table, again with Washington right side up, so the edge of the second coin touches the twelve o'clock position on the edge of the first coin.

Now the question: if you roll the second coin halfway around the edge of the first coin, from the twelve o'clock position to the six o'clock position, will George Washington come out right side up or upside down? 

You can try to figure it out. As you roll from twelve o'clock to six o'clock, exactly half the circumference of the rolling coin touches the edge of the coin you're holding down. So it seems that the rolling coin goes through half a turn, and Washington should come out upside down.

But when you try it, George Washington comes out right side up!

How can that be? If you roll a 25-cent piece along a “straight” edge, such as a ruler, for a distance equal to half its circumference, Washington comes out upside down, because the coin rolls half a turn in that situation. But when you roll one 25-cent piece halfway around “another 25-cent piece,” the curvature of the edge of the coin you're holding down with your finger adds another half-turn. The two half-turns add up. The rolling coin in our demonstration actually goes through one full turn.

And that's how George Washington comes out right side up. 

More like this:

• Underwater Money
• Kids and Money
• Flipping a Coin

Mentioned, among other places, in R.J. Brown, “333 Science Tricks and Experiments” (Tab, 1981), and Martin Gardner, “Entertaining Science Experiments with Everyday Objects” (Dover reprint, 1981).