So Mazur gave what he thought was a thorough and thoughtful explanation of the concept. He went slowly, putting all kinds of helpful diagrams up on the board.
"I thought I'd nailed it," he says. "I thought it was the best explanation one could possibly give of this question."
Mazur triumphantly turned around. "Any questions?" he asked. The students just stared at him.
"Nobody raised their hand and said, well but what if this and what if that, simply because they were so confused they couldn't," he says. "I didn't know what to do. But I knew one thing. I knew that 50 percent of the students had given the right answer."