A kunai knife is thought of as a popular ninja weapon as it could be hidden easily.

It is nothing more than a large, 10 to 15 centimeters long, flattened iron nail. It could be used as a climbing tool, substituted as a peg, dug the ground, or even thrown like a shuriken. It seems that it is a basic weapon, such as the shuriken.

According to some, and contrary to popular belief and anime histories, the kunai was not sharp, nor was it intended to be thrown. Kunai knives may not exist, and were not likely to have been used by actual ninjas or samurais.