In the 1950's, a few years before I was born, my grandparents were vacationing at the Grand Canyon, and saw a man taking photographs. My grandfather, who fancied himself a skilled amateur photographer, offered the man some unsolicited technical advice. The man listened politely, nodded, and thanked my grandfather courteously.

Some months later, they saw a series of breathtakingly good B&W photographs of the Grand Canyon in National Geographic magazine, and realised that my grandfather had been giving advice to Ansel Adams. My grandfather said that he had never in his life felt like such an idiot.