Have your sucker pull out a few coins and set them on the table. Count to yourself the total number of coins that are "heads" side up and not whether or not it's an odd number or an even number.

Next, turn around and ask them to flip over any coin they want. Then ask them to do it again. And again. Ask if they want to go again, then ask one more time if they want to flip another over. It doesn't matter how many times they flip over coins, as long as you know HOW MANY coins were flipped. To help you keep track, you can keep a "running count" of whether the current number is even or odd.

Once you turn around, note whether your running count is EVEN or ODD. Whatever your running count is, that's how many heads-up coins should be on the table. If you expect an even number, and you count an even number when you turn around: that means she's hiding a coin with TAILS side up. If you expect odd, but only count an even number of heads... then she's hiding the last heads-up coin!

Note: it's SUPER important that you practice your tempo and phrasing during the trick to make sure absolutely know how many coins are flipped over. If there's any ambiguity, your running count might get off and leave you looking like a chump at the reveal!