"A growing body of cognitive research, including Mr. Kornell�s study and a series of experiments presented at the American Educational Research Association conference earlier this month, suggests challenging material and study strategies�called �desirable difficulties��help students remember material better and longer. By contrast, trying to take the route that feels easier can lead students to develop study habits that they believe are helpful, but that actively interfere with their learning...

For a teacher trying to design an assignment, the ideal thing is to put your students in a situation where they are challenged. The more someone struggles with something, the more they are going to learn,� Mr. Kornell said. �You want them to eventually feel something is easy to process, but only because they�ve worked through it and made it their own, not because you made it easy for them.�

Robert A. Bjork, the director of the Learning and Forgetting Lab at UCLA, calls this sort of challenge �desirable difficulties.� Just as in physical exercise, the more students have to exert their mental muscles to learn a new concept or recall an idea, the stronger their memory and learning will become.