What is the traditional Japanese art of paper folding?

Origami, from ori, to fold, and kami, paper. It began as ceremonial - folded paper marked offerings at Shinto shrines and accompanied gifts - and paper was expensive enough that recreational folding only became widespread in the Edo period.

It has become a serious branch of applied mathematics, which is not a joke. The Miura fold, developed by a Japanese astrophysicist, lets a large flat sheet be deployed and retracted by pulling two corners, and has been used for satellite solar arrays. Related techniques have gone into a heart stent that unfolds inside an artery, airbags that pack predictably, and the folding of the James Webb Space Telescope's mirror and sunshield to fit inside a rocket. Robert Lang left a career at NASA to work on origami full time.

The thousand cranes carry a specific modern weight. Folding a thousand is traditionally said to earn a wish, and Sadako Sasaki, who developed leukaemia after Hiroshima, folded them in hospital before dying at twelve - so chains of paper cranes now arrive at the memorial there from all over the world, and the crane has become an international symbol of a wish against nuclear weapons rather than a private wish for anything.