Maple flowers are green, yellow, orange or red. The distinctive fruit are called samaras, "maple keys", "helicopters", "whirlybirds" or "polynoses". Seeds occur in distinctive pairs enclosed in a "nutlet" attached to a flattened wing of fibrous, papery tissue. They are shaped to spin as they fall and carry the seeds a considerable distance on the wind. During World War II, the US Army developed a special air drop supply carrier that could carry up to 65 pounds of supplies based on the aerodynamics of the maple seed.