During the 1973-74 academic year, Hawking was a visiting professor at Caltech, and I was a junior in the physics program. The first signs something was up were the wheelchair ramps that appeared around Bridge Laboratory, the older physics building. Hawking made his debut by giving the weekly physics colloquium talk in the large lecture room. At that time, he could still speak well enough to give a lecture in his own voice (with a microphone), and he started into some brand new, mind boggling stuff about black holes evaporating. The entire physics faculty was there, and I sensed he had zoomed over most their heads. Most, but not all. Feynman was sitting in the front row, and half way through, he asks a question. It was the kind of question that reveals he was following along just fine. The two titans proceeded to have a back and forth exchange for a few minutes in front of the assembled masses, and then Hawking continued with his talk. It was awesome and humbling to witness that, and my friends and I, sitting way up in the back rows, just shook our heads.