Japanese Type 91 grenade, 1931-1945 (c)

The Type 91 could be thrown as a standard grenade or fired from the 50 mm mortar. Removing the pin did not activate it, it only allowed the grenade to be armed. To start the fuse burning a soldier had to depress the moveable top portion of the head back into the body of the grenade. This often meant pushing it against a wall, tree or even against the thrower's helmet. Allied soldiers learned to recognise the metallic sound of a grenade being armed by striking the helmet and took this as a sign to take cover quickly.