James M. Lang, The Chronicle of Higher Education, June 3, 2012
"As the researchers pointed out in their article in the journal Cognition, both students and teachers may sometimes judge the success of a learning experience by the ease with which the learner processes or "encodes" the new information. But learning material easily, or fluently, may sometimes produce shallower levels of learning.
"By contrast, "making material harder to learn," the authors wrote, "can improve long-term learning and retention. More cognitive engagement leads to deeper processing, which facilitates encoding and subsequently better retrieval." In other words, when students encounter cognitive disfluency, and have to put in more work in processing the material, it may sink in more deeply."