A classic psychology experiment based in a simple, funny, yet devilish game shows how play reveals the inner working of our minds. Did you know that it’s rumored that a simple game helped counterintelligence agents discover Russian Spies?

I promise to get to the spies at the end; but first, here’s the game. Read the following set of words out loud:

It was this hesitation that caught the ear of an American psychologist, John Ridley Stroop, in 1935. American and German psychologists had long been interested in measuring the speed at which impulses traveled through the , and they became particularly curious about those factors that could inhibit or interfere with the transmission. Stroop noted that we have no trouble saying the name for the color when it is congruent with the ink color—when red is printed in red. But it takes longer to name the color of the ink when the word doesn’t match it—when the word “red” for example is, incongruously, printed in blue. (Black ink doesn’t seem to make a difference.) With this switcheroo you’re not just slowed down, you’re prone to error. You’re more likely to make a mistake and say “red,” when you’re supposed to answer “blue.” Psychologists, who have replicated the experiment in different versions many hundreds of times, now call the error-prone hesitation the “Stroop Effect” in honor of its discoverer.

Psychologists like the way that the Stroop Effect messes with your mind because the way the game interferes with the normal process helps them understand the normal process. So what is the normal process?

Ordinarily when we read the word “red” we think of the color red. We associate the text with the color naturally and automatically. When you play this game, though, you are asking your brain to override the powerful impulse to read for meaning—for semantic effect—and to detour from the accustomed neural pathways that your brain constructed when you learned to read. With a Stroop Test you’re challenged to make the irrelevant relevant.

Some claim that playing with the Stroop Effect isn’t just a brain-teaser; they say that discordant puzzles like these are so difficult that they become brain exercisers. If you believe this you can practice mental calisthenics with the Stroop Effect online.

Real life experience may present you with Stroop Tasks, too, as I learned from a classicist friend of mine who recently traveled to rural Albania to examine Greek inscriptions on ancient ruins. As it happens the custom of nodding yes is reversed there—in some areas Albanians still follow the Balkan habit of shaking their heads to agree and nodding to say no. For my friend, this U-turn ended with comic foreign entanglements—obliging but bemused waiters brought him increasingly larger tankards of ale and larger slabs of lamb that he thought he’d refused. When he discovered that some in this once isolated and now hospitable country had picked up the Western custom of nodding yes, the situation went from befuddling to baffling. One fellow became irate when my friend seemed to deny and then affirm that Mother Theresa hailed from Albania. Or was he only apparently irate? This might have been a game that the wry fellow enjoyed playing with tourists.

Finally, I promised to tell the story that circulates about the Stroop Effect and the great deadly game of espionage and counterespionage. I can’t say if it’s true or too good to be true.

But this much we know; deep cover spies can assume a new , dress like the natives, take on their mannerisms, master their foreign popular culture, and even convincingly root for the Philadelphia Phillies. Foreign spymasters can teach trainees to say “no way Jose” plausibly, and “Buddy, I tell you what,” and “what’s our game plan?” just like other Americans. Sleeper agents learn not to drop their definite articles and how to avoid other dead-giveaway linguistic traps. But such is the power of the mother tongue that spy-masters can’t teach even their best pupils a way to defeat the Stroop Effect. During the Cold War, American interrogators are reputed to have trapped native Russian-speaking spies by presenting them with a Stroop Task. The word красный, for example, means “red.” Show it written in blue to a monoglot American and you’ll get no reaction at all. But counterspies have their ways of detecting the Stroop Task hesitation, and could observe the momentary pause that gripped deep cover agents when showed the word красный in blue. In that tiny dithering interval, so the story goes, the Stroop Effect betrayed them.