It seems that numbers hold a special power for many of us. We may believe in lucky numbers, or look for numerological patterns in ancient texts and cryptic dispatches, perhaps from another world. For his part, legendary inventor Nikola Tesla reportedly had a numerical obsession of his own: the relationship he saw between the numbers 3, 6, and 9. The three digits—especially when found together—seemed to pique Tesla's scientific curiosity to the point that he called them the "key to the universe." While Tesla's fixation on these multiples of three clearly figured into his professional inquiries, it's thought that they also found their way into his eccentric personal rituals. He was known, for example, to use 18 napkins at a time—a number so chosen because it's divisible by 3, 6, and 9.