34. Playing the Odds
class: tweeners | impressiveness: 5/5 | factors: big risks, big rewards | requires: deck of cards | watch full episode
If you’re comfortable with taking a little risk for a chance at a HUGE reward, you will love these two impossibilities that rely on people’s inherent misunderstanding of probability.
5-card mind control: Let’s try a little mental experiment, just me and you. I’ve picked out five cards from the deck. Take a quick look at them, because in a moment I’m going to ask you to choose one. Before you do though, keep in mind that I might be trying to manipulate you into picking an obvious card, so think hard, take a moment, and choose one of the five cards above.
Got one? Don’t continue until you do…
Most people choose the seven of clubs or the four of hearts… But I’m going to guess you picked… the four of hearts. Was I right?
This is an old classic called Dai Vernon’s 5-card force. And it’s super easy to do:
In front of your friend, lay down (in this order) the king of hearts, the seven of clubs, the ace of diamonds, the four of hearts, and the nine of diamonds. Ask her to mentally choose any card. The key to this one is dropping just a hint that you might try to influence their decision. Once you mention that, they’ll eliminate the ace of diamonds as too obvious. They’ll eliminate the seven of clubs since it’s the only black card. They’ll eliminate the king of hearts as the only face card, and (for some reason), nobody seems to like the nine of diamonds.
In my personal experience, if they don’t pick the four of hearts, it’s usually the seven of clubs. So in performance, I’ll have them select a card, then I’ll scoop them all up and mix them around. Finally, I’ll pull out the seven of clubs and hold it face down in my hand as I ask “Were you thinking of the four of hearts?” If I’m right, the trick is over.
But if I’m wrong, I’ll ask “Hm… if you don’t mind telling me: what was your card?” If she says the seven of clubs, I’ll smile and turn over my “real” guess: the seven of clubs.
If, just now, you didn’t pick the four of hearts, don’t hold it against the trick. Obviously, it’s way different trying to perform a magic trick through text. Just try it as a throwaway trick a few times with your friends and see how well it’s received.
Side-by-Side: Next, a twist of probability that blew my mind when I first experienced it: grab a deck of cards and shuffle them. I’m serious: I want you to try this.
Now think of any two random card values (from ace through king, and don’t worry about suits). In your mind, what would you guess the odds are that two cards of exactly those values are right next to each other in your deck?
Are the odds 20%?
…10%?
…1%?
If we were sitting at the bar right now, would you bet a beer that those two numbers are more likely to be together or apart in the deck?
Amazingly (and to just about everyone’s disbelief), it seems that about 50% of the time, any two named values will just happen to be side-by-side in a shuffled deck of cards.
I now know that the odds are just a hair under 50%, because mathematician James Grime of Cambridge University just so happened to be a fan of scam school. And when he saw me incorrectly guess that the odds were 70% (which I assumed based on my handful of trials), he put together this awesome YouTube video calling me out and giving the correct statistics.