"In general, fire is used to throw enemies into confusion so that you can attack them. It is not simply to destroy enemies with fire. When you hear fire has erupted, you should then attack; once the fire has been brought under control and the people have settled down, it is no use to attack, so Master Sun says you should respond quickly" - Du Mu
A grenade is a small bomb typically thrown by hand. During the 14th century in China, the first small scale fragmentation bombs were invented. These differed from the earlier incendiary bombs.
They were made of a generally cast iron outer shell, which would be filled with small iron pellets, pieces of broken porcelain, and a mixture of heated oil which would coat the pellets. When the bomb was ignited by a small amount of gunpowder and a fuse, the resulting explosion sends flaming iron and porcelain shrapnel out, along with the iron pellets which would tear through the enemy and set them on fire.
To follow up the fragmentation bomb, the Chinese developed the hand grenade. The earliest descriptions of a hand grenade type weapon being used by Chinese soldiers is written in the Wujing Zingyao the same military text compiled by the Song to catalog the use and creation of the flame-thrower, gunpowder, and incendiary bombs. Within that regard the text states that during the Song dynasty soldiers used hand thrown explosives called “Zhen Tian Lei” which were small ceramic or metal pot like containers filled with gun powder, and thrown at the enemy. It is said that the Chinese developed the hand grenade off of an earlier Roman design. The Chinese would quickly evolve the hand grenade from a simply smaller version of their earlier cast iron incendiary bomb into the fully exploding fragmentation weapon we are familiar with today.
In Japan they were named Houroku-Hiya (焙烙火矢). A sphere with a circumference of about 120mm and a weight of about 500g. Inside the sphere was gunpowder, scraps of iron, and small nails that were tightly packed. Depending on the length of the match, shinobi could adjust how fast it exploded.