Sadako Sasaki was a Japanese girl who died of Leukemia at the age of 12, 10 years after the bombing of Hiroshima. While in hospital before her death, she heard an ancient Japanese story that anyone who folded 1000 paper cranes would be granted a wish. She believed that folding cranes would aid with her recovery and folded them until her death. In her memory, her friends and classmates started a fund to build the Children’s Peace Monument in Hiroshima. And her story has made paper cranes an international symbol for peace.
Children from around the world send approximately 10 million paper cranes to the monument each year.
This is a YouTube tutorial on how to fold the cranes. Be sure to practice a few times before showing the girls. It is also easier to teach origami in smaller groups so you might want to make it part of a round robin of activities.
More information on the park and instructions for sending your cranes can be found here.