It sounds unassuming, but spaced repetition produces impressive results. Eighth-grade history students who relied on a spaced approach to learning had nearly double the retention rate of students who studied the same material in a consolidated unit, reported researchers from the University of California-San Diego in 2007. The reason the method works so well goes back to the brain: when we first acquire memories, they are volatile, subject to change or likely to disappear. Exposing ourselves to information repeatedly over time fixes it more permanently in our minds, by strengthening the representation of the information that is embedded in our neural networks.

A second learning technique, known as “retrieval practice,” employs a familiar tool — the test — in a new way: not to assess what students know, but to reinforce it. We often conceive of memory as something like a storage tank and a test as a kind of dipstick that measures how much information we’ve put in there. But that’s not actually how the brain works. Every time we pull up a memory, we make it stronger and more lasting, so that testing doesn’t just measure, it changes learning. Simply reading over material to be learned, or even taking notes and making outlines, as many homework assignments require, doesn’t have this effect.

According to one experiment, language learners who employed the retrieval practice strategy to study vocabulary words remembered 80 percent of the words they studied, while learners who used conventional study methods remembered only about a third of them. Students who used retrieval practice to learn science retained about 50 percent more of the material than students who studied in traditional ways, reported researchers from Purdue University earlier this year. Students — and parents — may groan at the prospect of more tests, but the self-quizzing involved in retrieval practice need not provoke any anxiety. It’s simply an effective way to focus less on the input of knowledge (passively reading over textbooks and notes) and more on its output (calling up that same information from one’s own brain).

Another common misconception about how we learn holds that if information feels easy to absorb, we’ve learned it well. In fact, the opposite is true. When we work hard to understand information, we recall it better; the extra effort signals the brain that this knowledge is worth keeping. This phenomenon, known as cognitive disfluency, promotes learning so effectively that psychologists have devised all manner of “desirable difficulties” to introduce into the learning process: for example, sprinkling a passage with punctuation mistakes, deliberately leaving out letters, shrinking font size until it’s tiny or wiggling a document while it’s being copied so that words come out blurry.

Teachers are unlikely to start sending students home with smudged or error-filled worksheets, but there is another kind of desirable difficulty — called interleaving — that can readily be applied to homework. An interleaved assignment mixes up different kinds of situations or problems to be practiced, instead of grouping them by type. When students can’t tell in advance what kind of knowledge or problem-solving strategy will be required to answer a question, their brains have to work harder to come up with the solution, and the result is that students learn the material more thoroughly.

Researchers at California Polytechnic State University conducted a study of interleaving in sports that illustrates why the tactic is so effective. When baseball players practiced hitting, interleaving different kinds of pitches improved their performance on a later test in which the batters did not know the type of pitch in advance (as would be the case, of course, in a real game).