Tibetan and Zen Bhuddism were completely separate for, I don't know, thousands of years. The professors at Harvard (I believe (a lot of "believes" herein, no doubt)) decided it would be fun to bring a master of each tradition to a seminar. They thought the ensuing discourse would be a delight and very much the thing for wine and cheese in the aftermath. Well, these traditions are rather different. Zen is more severe, and its practitioners like to cast doubt like a ratan cane on everything. Came the evening and the masters were seated opposite each other. The Zen master held an orange out before the Tibetan and demanded, "What is this?" When the Tibetan didn't answer, the Zen master demanded again, "What is this?" And so on. Finally the Tibetan took the orange and said, "It's an orange, don't you have them where you come from?"

embellished from hazy memory