"Those who can, do. Those who understand, teach." -- Lee S. Shulman, "Those who understand: Knowledge growth in teaching", Educational Researcher vol. 15, no. 2, February 1986 (pp. 4-14)
( additional context and a longer form of the quotation, for the curious )Wikiquote notes that the short form is often attributed to Aristotle -- and that's how I first saw it, but it didn't sound right, so I went to Qikiquote to check, and found there a helpful link to the paper it's from.