Asked by Todd L Ross in Coins and Paper MoneyMagic and IllusionsStatistics
How do you flip a coin?
August 21, 2019 4:17PM
Here’s the basic process. After you’ve got this down, we’ll look at a few ways to influence the outcome of the coin flip.
- Step One - Make your hand into a fist, wedging your thumb against your index finger or in the crease between your index finger and middle finger. Point the thumb side up.
- Step Two - Place the coin on top of your fist on the space between your thumb and index finger. You can use any coin, but larger coins work better.
- Step Three - Send the coin into the air by pulling your thumb up. Don’t snap too hard; a soft toss is perfectly fine. As the coin goes into the air, open your palm to prepare for the catch.
- Step Four - Catch the coin. This will be much easier if you keep your tosses relatively soft. Provided that your thumb was properly positioned, the coin shouldn’t move too far in the air, but you’ll need some practice to perfect your technique.
You should be able to learn a basic coin flip in 10-20 minutes, but if you want to actually force the coin flip to go your way—or if you want to be able to guess a coin flip consistently to impress your friends—you’ll need to learn a few simple sleights of hand.
Changing the Outcome of the Coin Flip
There are dozens of ways to alter or predict the outcome of a coin flip. We’ll focus on two of the easiest methods:
Feel the coin when you catch it. With some practice, you can easily differentiate between the smooth “heads" side and the comparatively rougher “tails" side. If the coin’s in your palm and you’ve got an outcome you don’t like, flip it once more while displaying it.
The classic method, of course, is to flip it onto the back of your opposite hand, but people will catch on to that pretty quick.
If you move your hand while displaying the coin, you may be able to flip it with your thumb during the display. This is a bit tricky, but with practice, it’s an easy trick to pull on your gullible friends.
Don’t actually flip the coin. Instead of flicking your thumb to flip the coin, use your index finger to give it a slight wobble as you toss it softly into the air. Simply pull your index finger along the edge to spin the coin like a frisbee.
The wobble will make the coin seem like it’s flipping, but it’ll actually stay in the same position, so if it starts as “heads,” you simply need to call “heads.” This takes some practice, but it’s remarkably effective.
How random are coin flips, anyway?
Let’s say that you’re not interested in learning sleights of hand. Could you gain any sort of advantage by picking “heads" over “tails,” or vice-versa?
Possibly. Statistician Persi Diaconis worked with Harvard University engineers to study coin flips and found that if a coin starts out “heads,” it ends up “heads" slightly more often than it ends up “tails.” Because humans flip coins in very different ways—using different angles and speeds—it’s difficult to set exact odds, but Diaconis found that “for natural flips, the chance of coming up as started is about .51.”
That assumes that the coin toss is perfectly normal. As coins age, they might wear down on one side, resulting in even less randomness, and if a coin spins on its side—as often happens when a coin is flipped onto a surface—its outcome can be relatively predictable.
“It’s easy to find coins which are 80 percent tails,” Diaconis says in one video.
Those coins tend to have un-serrated edges (so quarters are safe). As the coin hits the ground or another hard surface—not your palm, which stops the movement—it tends to balance on its edge for a moment, at which point it falls to its preferred side.
How can you make sure that a coin flip is fair?
With all of that in mind, here are some rules for ensuring that a coin flip is as random as possible:
- Don’t allow the flipper to set the coin’s position. The coin’s position has a slight effect on the outcome (the coin’s starting position is 1 percent more likely to be the final position).
- Use a coin with a serrated edge. This will limit the chances that the coin will fall to a particular side if it hits a surface.
- Try to use a relatively new coin. This limits the chances that wear on one side of the coin will influence the flip.
- Make sure the flipper catches the coin. When coins hit hard surfaces (or the ground), they’re generally more likely to land on a particular side. Every coin is different.
- Don’t allow the flipper to flip the coin after catching it. This prevents the flipper from choosing the outcome they want.
That’s pretty much everything you need to know about coin flips. Remember, you shouldn’t use your new skills for anything dishonest—just have some fun tricking your friends and family.