On an early autumn morning in 2009, Randall Munroe, a NASA physicist turned full-time cartoonist, was teaching a weekend physics class to high-school students in Cambridge, Massachusetts. The course was part of an M.I.T. program designed to introduce students to topics ranging from sculpture and ancient Greek to geoengineering. Though Munroe’s lecture that day had the lively title “Solar Panels, Hand Grenades, and Blowing Up the Moon: How to Think About Energy,” for the first hour and a half he adhered to a fairly standard lecture format. What is energy? What can it do? How do you know how much you have?

The students reacted as, well, typical students. “They seemed pretty bored,” Munroe recalled. “I could tell, because I remembered being that bored student.” Maybe, he joked, one of them had even begun doodling—his own preferred method for passing time when he was in school, and a habit that would later evolve into his popular online cartoon strip “xkcd.”

Halfway through his lecture, Munroe decided to shift tactics. Veering away from the traditional classroom approach, he began illustrating concepts with zany examples. What if, for instance, the students had to sort out the energy dynamics of a scene in “Star Wars” or “The Lord of the Rings”? “Suddenly, the kids were excited and engaged,” he told me. “Before I knew it, they were running ahead of me, coming up with their own examples and solving their own equations.” At that moment, he realized that formulating exciting, relevant questions—questions that stemmed from students’ own concerns and interests, however far removed from a lecture hall—might sometimes be the best way to help people understand disciplines as complex as physics. However absurd and hypothetical, such questions seemed to engage students’ minds in a way that simple formulas alone did not.

* * * When we attempt to learn something new, our

level of interest in the material is one of the best indicators of how successful we will be. If we are bored by calculus, muddling through on a combination of memorization, technique, and brutal willpower, we may well end up earning the exact same grade as someone who sees the equations as beautiful illustrations of how the world works. But ask us five years—or even five months—down the line to tackle a problem or explain a concept, and those of us who lacked genuine interest will be far more likely to draw a blank than the truly curious. Interest not only makes us more attentive and focussed but also seems to increase the cognitive resources we bring to a subject: we throw much more brainpower at problems that intrigue us than at those that don’t. But what is it that creates interest to begin with?

For a long time, researchers believed that interest was relatively stable. Some people were simply more drawn to certain topics than to others: you either loved physics or didn’t much care for it, and, while a good teacher could perhaps pique your interest, she couldn’t change your underlying disposition in the long run. (Your disposition could, though, be influenced by other aspects of your personality: some people, for instance, were broadly curious and engaged—they seemed to be interested in everything—while others were more motivated or achievement-oriented. Those traits could inspire learning even in the absence of overwhelming interest in a topic.) And yet, we’ve all seen counterexamples to this theory of stable interests: there’s the indifferent student who hates science, encounters a brilliant chemistry teacher in high school, and goes on to get her Ph.D. in biochemical engineering. Or the student who sees English as a waste of time, only to come across a book that hooks her on literature for life.

Munroe recalls that he wasn’t necessarily as attentive as he should have been in his own physics classes. “I remember at one point making a careful table of how many people each type of dinosaur had eaten in ‘Jurassic Park’—both the book and the movie, of course, because the list of who dies has some major differences.” And, although he’d liked science as a kid, he spent several years feeling unengaged by his school curriculum. “At some point, possibly after dissecting a squid, I started thinking that maybe I didn’t like science after all,” he said. He regained his enthusiasm only after a tenth-grade science teacher pointed him to a physics book when she couldn’t answer a question he’d posed about radioactive decay. “I started flipping through it, and saw page after page about light, batteries, cranes, stars, waves, particles, electricity, and more,” he recalled. “And I remember thinking, Oh, this is where the science I like is.”

Recent research has explored how, exactly, interests evolve over our lifetimes. In a meta-analysis of sixty-six studies tracking interests over time (the average study followed subjects for seven years), psychologists from the University of Illinois at Urbana–Champaign found that our interests in adolescence had only a point-five correlation with our interests later in life. This means that if a subject filled out a questionnaire about her interests at the age of, say, thirteen, and again at the age of twenty-one, only half of her answers remained consistent on both. And while people’s interests in college were more stable, correlating at just under point seven with their interests later in life, they, too, remained substantially malleable. What’s more, the researchers found that interest in some fields, such as sports, was far more stable than interest in others, such as science.

What is it, then, that can capture our curiosity about subjects that once bored us? As it turns out, the approach Munroe tried out at M.I.T.—the absurd hypothetical—is exactly the kind of technique that we now know is likely to foster engagement. He has collected some of these hypotheticals in “What If?,” an illustrated book of outlandish physics questions, out this month, that investigates serious scientific concepts. Could we really live, like the Little Prince, on an asteroid if it were big enough? Is it possible to build a jetpack using downward-firing machine guns? Even someone who has never given much thought to concepts like thrust-to-weight ratios may be suddenly drawn into a world where AK-47s (“a minimum of 25 but ideally at least 300”) enable us to lift off into space.

* * * In 1966, the University of Toronto psychologist Daniel Berlyne

proposed that curiosity arises when we encounter uncertainty or ambiguity in our environment, whether it’s physical (a place or an object we haven’t seen before) or mental (a word we don’t know, a question we can’t answer). Any time our experiences don’t quite correspond to what we’ve previously encountered—a phenomenon that the Carnegie Mellon psychologist George Loewenstein later called the “information gap”—we start to pay attention. But uncertainty and ambiguity alone aren’t sufficient to generate sustained interest over time. You may not know whether the sun revolves around the Earth or what that orange thing on the sidewalk is, but you may not particularly care, either. What’s needed beyond a gap in knowledge, Berlyne argued, is incongruity and surprise: something that seems to outright contradict what we already know, or to be otherwise extremely novel. When new ideas or experiences somehow don’t mesh with old ones, we react with curiosity.