The crane is the international symbol of peace.

In Japan every child eventually learns to make the crane. Eleanor Coerr wrote a book "Sadako and 1,000 Paper Cranes." This book tells the story of a young girl who was exposed to the radiation from the atomic bomb that the U.S., dropped which ended World War Two. Several years later she develops leukemia. 

Her friend visits her in the hospital with an origami crane. She tells Sakako that the crane is a symbol of health and that if Sadako can make 1,000 cranes she will be well. Her friend proceeds to teach her to make the crane: it isn't easy but when Sadako masters it, she begins her quest to make 999 more. She is resolved to be brave and making the cranes takes her mind off her illness.