Cranes are said to live for 1,000 years, which is why folding 1,000 of them brings good luck, happiness, or a single wish. Senbazuru, the tradition of folding a thousand cranes, became popularized by the story of Sadako Sasaki, who folded 1,000 cranes when they found out they had leukemia due to radiation exposure, a result of the US dropping an atomic bomb on the city of Hiroshima. Sasaki is also a symbol for the devastating effects of nuclear war.