On a Saturday afternoon the week before Christmas in 1935, I walked along Nassau Street in Princeton, New Jersey, looking at the displays in shop windows and daydreaming about a certain high school girl. I soon reached the new F. W. Wool-worth five-and-dime, the only self-service store in Princeton at the time.

Displayed in the front window was a toy, a little wheeled cart on a steeply inclined ramp. The cart was attached to a cord that ran over a pulley at the top of the ramp; at the end of the cord was a counterweight. Perched above the top of the ramp was a hopper full of sand. When the cart was empty, the counterweight pulled it to the top of the ramp, where it tripped a lever to start sand funneling into it. Once the cart filled up, it became heavier than the counterweight and rolled back down the ramp, shutting off the sand flow from the hopper as it departed. When the cart hit the bottom of the track, it did so with enough force to tip-dump the sand. Then the counterweight pulled it back up the ramp for a refill. This went on and on.

I stood for several minutes studying this fascinating toy. Soon I sensed that someone was standing beside me, and when I glanced to my right, I saw a slightly bent man with a mustache and gray, frizzy hair. He was wearing an open-collared white shirt, baggy khaki pants without a belt or suspenders, and ankle-high tennis shoes without socks. He seemed underdressed for December.

I knew who this was. He had come to Princeton to hold a post at the Institute for Advanced Study, and I had seen his picture in newspapers and magazines, and on newsreels. I was afraid he might say something that my 15-year-old brain wouldn’t understand.

For a while he just stood there watching the toy. Then, just as I feared, he started talking to me.

“This is fascinating, isn’t it?”

“Yes, sir,” I said, “it certainly is.”

“The people who invent these modern toys are ingenious,” he said. “This clever little machine nearly accomplishes perpetual motion.” Then Albert Einstein turned and walked away.