These findings, among others, indicate that students forget less than they may think they do. And there’s value in what they remember. These conclusions carry important implications for the subject matter students study in school.

Naturally, knowledge sticks if it’s revisited. For example, one study of MIT students found that physics majors remembered material from a freshman course better than students who majored in subjects unrelated to physics. More striking, though, is that continued use can actually make knowledge indelible. In one rather remarkable study, researchers administered an algebra test among adults who had taken algebra anywhere from months to decades previously. Most of the adults struggled to remember how to do the equations, but those who’d studied math beyond calculus (subjects whose mastery requires an understanding of algebra) could still work basic algebra problems—even if they had not done so for decades. In other words, several years of practicing algebra in more advanced math courses made the former stick permanently.

So why do adults remember some facts they learned in school but not others? For one, the context of a memory—where and when it’s learned—might be forgotten even if the content is recalled. That’s what happens when one recalls hearing a movie is good, but can’t remember who said so. Likewise, a student may remember a fact but not know she learned it at school. And if she hears the same fact many times, figuring out where she learned it first can be especially hard; who first told her that there are four quarters to a dollar? A parent? A teacher? Someone on Sesame Street?

Other times a student remembers the context—he knows he studied French at school, for example—but falsely concludes that he’s forgotten everything. After all, it’s likely that some of the memory remains even if he recalls nothing. This invisible residue of old memories helps a person remember that same material again more quickly than before. Clever research studies on this phenomenon tested Mormon missionaries who learned a foreign language but didn’t use it again for decades; forgotten vocabulary was quickly relearned. Other research in more controlled laboratory situations showed comparable results.

Ultimately, this ability to retain some of that knowledge has practical benefits—and the reason for that has to do with the nature of intelligence.

Intelligence has two components. One is akin to mental horsepower—how many pieces of information a person can keep in mind simultaneously, and how efficiently that person can use it. Researchers measure this component with simple tasks like comparing the lengths of two lines as quickly as possible, or reciting a list of digits backwards. The other component of intelligence is like a database: It entails the facts someone knows and the skills he or she has acquired—skills like reading and calculating. That’s measured with tests of vocabulary and world knowledge.