Cutaway of the British Model 36 Mills Bomb
Oddly enough, this WWI grenade did not come ready to use like modern grenades. Each Mills had to be assembled by the user before they could be hurled against the enemy. Each battalion had a "Bombing Platoon" that specialized in explosives, mostly Mills Bombs and Gun Cotton.
Still, it was a step up from the Grenade No 1 first issued to British troops in the opening months of the war which were a cast-iron canister on an 18 inch stick.
Soldiers soon discovered that they were dangerous to use when in a front-line trench. There were several cases of soldiers being killed when the grenade hit the front of the trench and bounced back onto them.