There was this movie called Musa:The Warrior whereby a Korean slave saved the Zhu Yuanzhang's sister (played by Zhang Ziyi) from the mongols. In a scene where Zhang Ziyi and civilians were in an abandoned city and the mongols were attempting to knock down the gates and couple of civilians threw down an "black iron ball" with a string attached to it. Of course, they ignited the string and when the string burnt up and the sparks hit the "black iron ball", it exploded.
Can anyone explain what is this black iron ball with the string attached to it?