Albert Einstein spent his last two decades trying to reconcile quantum physics with relativity. His holy grail -- a so-called "Unified Field Theory" -- eluded him. He once casually mentioned to a colleague that he was on the verge of his "greatest discovery ever," before admitting that "it didn't pan out" just two weeks later.

One day in his twilight years, he received a letter from a 15-year-old girl asking for help with a homework assignment. She soon received a curious reply: a page full of unintelligible diagrams, along with an attempt at consolation: "Do not worry about your difficulties in mathematics," Einstein told her. "I can assure you that mine are much greater!"