On Harvard Square, back in the mid-l930s, there was a small thin eating-place frequented by four of us graduate mathematics students. We called it The Slit in the Wall, and one day a surprising mathematical event took place there—in our favorite booth at the very rear of the place.

We had each enjoyed a big bowl of delicious minestrone soup when I casually announced that I had discovered an “obvious” proof of a very teasing relationship in number theory that had been eluding us for some time. The word “obvious” caused a ripple of surprise among my friends, along with a feeling of considerable skepticism.

“No,” I insisted, “my proof is truly obvious, because the relationship can immediately be seen by merely looking at a simple geometrical diagram.”

These words only increased the curiosity and incredulity of my friends—for what could a simple geometrical diagram possibly have to do with a problem in number theory? I produced a pen and asked if someone had a piece of paper. A piece of paper was not forthcoming, as we all, according to custom, had left our books and school things back at Sever Hall while we went to lunch. It was then that we noticed the proprietress of the establishment standing at our table, amused by the commotion we had made. We were all very fond of her—she had long given us special treatment and affectionately referred to us as “my mathematicians.”