I once asked a teacher what he like most about his branch of mathematics. He replied that it was the beauty and simplicity of theorems. Such as what? I asked. His eyes flashed with enthusiasm. “Such as the proof that the intersection of two quartics is a twisted cubix.” Seeing a glazed look in my eyes, he began to sketch the proof. I held up a hand, laughing, and said, “Wait a minute. I’ve never even heard of these things.” But, it was too late. The teaching fit was on him. He began to explain. I was perplexed and I said so. He began to grow exasperated-like most teachers when their “explanations” are not being understood. “It’s really very simple!” he said, as his hands made complicated shapes in the air. I was amused but also appalled. There was an experienced teacher who believed so strongly in the magical power of explanation that he thought he could drop me into the middle of an advance and complex branch of mathematics in which I had absolutely no knowledge or experience, and with a few words and waves of the hand make the whole things perfectly clear.