He had a dream — a logical puzzle

By Frederic Friedel

There are logical puzzles that are very difficult, and some that are trivially easy. But here is one that is both. At least in my experience. I have given it to a substantial number of people — about half of them look at me in bafflement and do not know what the question is. The solution is so obvious to them that they have to see someone ponder over it for an hour to believe that anyone could not see the solution instantly. The other half — well, they ponder over it for an hour, or even days or weeks, in some cases.

Here’s the problem — you can find out to which half of the world you belong.

Recently a friend told me the following story about the death of his grandfather: “My grandparents used to go to church on Sundays. One day during the sermon, which was long and dry, my grandfather fell asleep. That week he had been reading a novel about the French Revolution and began to dream that he was a rich aristocrat living in a beautiful chateau in France.
Suddenly there was a commotion outside. A mob of peasants appeared, stormed the house, grabbed him and tied him up, dragged him to the market square, where there was a platform with a guillotine set up. My grandfather was led up the stairs, a priest muttered a few words to him and then his head was placed in the cradle of the guillotine. A hooded executioner approached and reached up for the lever that releases the blade.
At that moment my grandfather was snoring loudly, so my grandmother reached out and pinched him on the back of his neck to wake him up. This was such a shock to my grandfather that he suffered a heart attack and died on the spot.”
My reaction to the story of my friend: “I don’t believe a word of that. You made it up.” Why did I react in this way, how did I know the story is not true?

If you belong to the 50% of the population who get it immediately, try it on other members of your family, on friends and acquaintances. You will be amazed how thoroughly stumped some of them can be. Like the world-class chess player, whom out of politeness we will not identify. I gave him this puzzle during a Japanese lunch, and he mulled over it all through the Miso and Sushi. We discussed the history of the French Revolution, the mechanics of the guillotine and the height of the lever. At one stage he made me repeat the puzzle in order to cleverly identify minor deviations from my original version (“That way I know that those details are irrelevant!”). As the meal came to an end he was becoming unpleasant and rude, so I had to tell him the solution. He sat there in stunned silence. How could someone who was clearly one of the most intelligent and logical people alive, fall for something as simple as this?

Some years ago I published the puzzle on my chess news page, and it was solved by a large number of readers, many of whom expressed disbelief that anyone would not be able to see the solution instantaneously. I also received a number of wrong solutions, some of which were quite convoluted. For instance the following: “If someone is sleeping, their heart beats at a slow rate. A pinch cannot instantly influence the action of a beating heart, so it is not possible that the shock of the pinch could have resulted in an instant heart attack death.”

No, that is not even close. Remember that I knew for certain that my friend’s story was not true. This is because it contains a glaring logical error. Once you hit upon that you know for certain that you have the correct solution.

So what is the solution already? Unfortunately (for some of you) I cannot bring it over myself to reveal it. As we mentioned before, an easy way to solve the problem is to give it to friends or family (tell them not to blurt out the solution if they find it). About half will be frowning and plying you with questions, while the other half will have a big grin or puzzled smile on their faces. Ask one of them to whisper the solution into your ear.