The kunai blade was a popular ninja weapon, it could be easily hidden and, because of their small size and effectiveness, could be removed quickly in combat.
The kunai served several purposes, but primarily it was used as a knife; the tip was sharp with serrated edges, also used for probing, digging and chiseling and even to open doors or any type of locks. People tied it at the end of a thin string, in order to be released, and due to its aerodynamic shape, when thrown, always hits with the tip.
The kunais are known as the typical tool of the ninja in the Naruto series, and in popular culture is often said that the kunai was a very versatile weapon and that the only impediment to use it was the imagination of the wearer.