Two secret agents need to meet for the very first time. They don’t know anything about each other—can’t even be sure that they live in the same hemisphere—and they will recognize the other agent when the day comes, but that’s about all.

The spies haven’t previously agreed on anything, and they can’t communicate in any way, neither directly nor indirectly. The deadline for the meeting is one year. Within 12 months, the two agents need to meet without sharing so much as a tweet.

Where and when will they meet? And what would you do if you were one of the agents?

I love to use this puzzle as a job interview question. It’s obviously “unsolvable” in a way that no two people will come to the same conclusion. Rather, it is an opportunity to observe creative problem-solving in real-time and to see how candidates talk through different approaches.

To maximize the chance of having a successful meeting, the agents need to think through what they themselves would do in such a situation, as well as imagine what the other agent might do. Since there’s no prior agreement and no knowledge of each other’s whereabouts, their best hope is to increase the probability of a chance meetup by choosing a place and time that most other agents would reasonably think of.

I don’t know much about spies, but I know that when I lived in Berlin, Germany, I only ever met up with friends who specifically came to visit me. On the other hand, when I lived in Manhattan, I only needed to wait, and sooner or later, all my friends from all corners of the world would eventually come by. So, “my” agent would choose New York for the meetup, and if it’s New York, it has to be Times Square, and if it’s Times Square, it’s probably New Year’s Eve. In my line of thought, the whole ball drop thing is just one massive secret agent meetup.

Life is both chess and poker.

In chess, anyone can observe the state of the board. Nothing is hidden: Both players can see all rooks at all times, and if they know who’s up, they have all the information they need about where the game stands. There’s little room for chance in chess. The outcome of the match is determined solely by the decisions of the two players.

In contrast, poker players have partial information about the game. Each player knows their own cards, but they don’t know what their opponents are holding. A player’s goal is to make the best possible decision based on their own cards and the information they’ve gathered from the betting patterns and behavior of their opponents. Successful poker players are generally skilled in both strategy and psychology, as they must read their opponents’ behavior to try and deduce what cards they might be holding.

Some of the problems we encounter in life will be like chess, where everything we need to solve them will already be included in the text. A telltale sign that identifies these kinds of problems is that we can change out the main words or objects, and we’ll have a similar problem with a similar solution—a mathematically isomorphic problem, just like in elementary school, in which word problems were translated into arithmetic tasks.

However, other puzzles will require outside knowledge. One great example is “the bus riddle”: The teacher first shows a schematic drawing of a bus. The image was created by a lazy artist who didn’t draw the inside of the bus, making the whole thing symmetrical so both ends of the bus look exactly the same.

Which way is this bus going?

Source: Drawing by Richard Dancsi

Our job is to find out which way the bus is going.

To solve a riddle of this kind, we need to know a bit about the world. Here, we need to know how buses work: where the driver sits, what happens at a bus stop, which side of the road buses drive on, and so on. The key to the solution is to know that buses always stop at the bay on the right-hand side of the road—therefore, that’s the only side of the bus that will have doors.

The riddle that knew too little

Which riddles people find easier to solve depends on the cultural context as well. One of the most well-known tasks in the study of deductive reasoning, the Wason selection task, asks participants to solve this puzzle:

You are shown a set of four cards placed on a table, each of which has a number on one side and a colored patch on the other side. The visible faces of the cards show 3, 8, red, and brown. Which card(s) must you turn over in order to test the truth of the proposition that if a card shows an even number on one face, then its opposite face is red?

As it happens, the correct answer can be found much more easily by simply relabeling the cards. Evolutionary psychologists Leda Cosmides and John Tooby found that by relabeling the cards as “drinking beer,” “drinking soda,” “age 16,” and “age 25,” people were able to solve the puzzle much more quickly. For instance, we need to know who was drinking beer and what the 16-year-old had, and we already know that anybody can drink soda, and 25-year-olds can drink whatever they please.

As we navigate the world, we solve riddles every day without even noticing. The only problem is that we don’t always use the right tool for the job. When a problem looks like a chess game and comes pre-packaged with all the information we need to reach a solution, the rational part of our brain might be best placed to help. And when it looks more like we’re sitting at a poker table, relying on our intuition, our emotions and previous experiences can fill the gaps in our knowledge.