According to an ancient Chinese legend, one day in the year 240 B.C., Princess Si Ling-chi was sitting under a mulberry tree when a silkworm cocoon fell into her teacup. When she tried to remove it, she noticed that the cocoon had begun to unravel in the hot liquid. She handed the loose end to her maidservant and told her to walk. The servant went out of the princess's chamber, and into the palace courtyard, and through the palace gates, and out of the Forbidden City, and into the countryside a half mile away before the cocoon ran out. (In the West, this legend would slowly mutate over three millennia, until it became the story of a physicist and an apple. Either way, the meanings are the same: great discoveries, whether of silk or of gravity, are always windfalls. They happen to people loafing under trees.)