Why does learning that feels harder stick better?

Because the feeling of learning and actual learning are different things. Methods that feel smooth, like rereading your notes, produce quick familiarity that we mistake for lasting knowledge. Methods that feel effortful, like testing yourself, spacing out sessions or mixing topics, slow you down in the moment but leave far more behind. The UCLA psychologist Robert Bjork named these 'desirable difficulties' in 1994.

The trap is that our own judgment points the wrong way. In a study by Nate Kornell and Bjork, people learned to recognize painters' styles from six paintings each by twelve little-known artists. Seeing one artist's paintings in a row felt better, and an overwhelming number of participants preferred it, yet they were significantly better at identifying the painter of new works when the paintings had been mixed up.

Testing works the same way. Restudied material often looks better on an immediate quiz, but material you had to retrieve wins as time passes. William James noticed this back in 1890: when you almost know something, it pays to recollect it by effort rather than look at the book again.

Not every difficulty is desirable, though. A task has to remain doable: too hard and the learner gives up. And most research spans only hours or days, so the long-term picture is still being filled in.