But it’s good to know about your environment even if it doesn’t promise a reward right now; knowledge may be useless today, but vital next week. Therefore, evolution has left us with a brain that can reward itself; satisfying curiosity feels pleasurable, so you explore the environment even when you don’t expect any concrete payoff. Infants prefer to look at novel pictures compared with familiar ones. Preschoolers play longer with a mechanical toy if it’s difficult to deduce how it works.

What’s more, curiosity doesn’t just ensure new opportunities for learning, it enhances learning itself. In a recent experiment, subjects read trivia questions and rated how curious each made them feel. Later, they saw the questions again, each followed by a photograph of a face, and judged whether that person looked like he or she would know the answer. In a surprise final memory test for the faces, subjects better remembered those appearing after trivia questions that made them curious. Curiosity causes a brain state that amplifies learning.

This function of curiosity — to heighten memory — is the key to understanding why we’re curious about some things and not others. We feel most curious when exploration will yield the most learning.

Suppose I ask you, “What’s the most common type of star in the Milky Way?” You’ll obviously feel no curiosity if you already know the answer. But you’ll also feel little interest if you know nothing about stars; if you learned the answer, you couldn’t connect it to other knowledge, so it would seem nearly meaningless, an isolated factoid. We’re maximally curious when we sense that the environment offers new information in the right proportion to complement what we already know.

Note that your brain calculates what you might learn in the short term — your long-term interests aren’t a factor. That’s why a cardiac surgeon who is passionate about her job will nevertheless find a conference presentation on the subject boring if her brain decides that the talk won’t add to her knowledge. Conversely, when a friend persuades her to watch a documentary on type fonts, her brain may calculate that this will be a rich source of information — and she finds herself fascinated.