The Mills Bomb was the standard hand grenade of the British Army throughout most of the 20th century
, so called because it was designed by a man called William Mills
of Birmingham, in 1915. The Mills Bomb was a 'pineapple-style' fragmentation
grenade, consisting of an explosive core around which was wrapped a thick, knobbly metal case; when the bomb exploded, the force of the explosion blew the case apart, sending shrapnel
in all directions. In an enclosed space - such as a trench dug-out - the mixture of supersonic metal fragments and concussion
Up until then the standard British hand grenade resembled an enlarged version of the WW2 German 'potato masher' - an explosive cylinder on the end of a long stick. These primitive grenades were awkward to use and manufacture, however, and they couldn't be fired from the end of a rifle.
The Mills Bomb, on the other hand, was small, could be held and used with one hand, and with a special attachment the grenade could be launched from a Lee-Enfield. William Mills designed the bomb in 1915, after a year and a half of increasingly desperate trench warfare had left the British Army in need of more effective grenade. There were never enough Mills Bombs to go around, however, and even at the end of WW1 the old-style grenades were still in use. Indeed, so short was the supply of grenades, and so handy were they, soldiers often took to improvising explosions with guncotton, biscuit tins, hobnails, stones and whatever came to hand. These improved devices were a danger to all concerned; conscious of the risk of accidental explosion, William Mills devised a clever ring-pull arrangement which is still used in hand grenades nowadays.
The Mills Bomb continued as the standard British grenade through World War 2 and into the mid-80s, by which time it was called the L2A2. In line with the rest of NATO, however, the British Army eventually replaced the Mills Bomb with a smaller, lighter grenade, the Swiss HG85, one designed with more emphasis on penetrating body armour. The Mills Bomb is now thought of as overkill, especially as the indiscriminate imprecision of grenades now has a much lesser role in warfare, at least in those conducted by the Anglosphere.
It's worth noting that, until WW2, the standard fuse on a Mills Bomb was seven seconds long, a length chosen in order for rifle-launched grenades to have time to fall back to earth. Unfortunately, this delay meant that it was relatively easy for enemy soldiers to throw the grenade back. The only way to prevent this was to pull the pin, wait, and then throw the grenade; but variations in manufacturing techniques and individual timekeeping made this risky. By WW2 the fuse was shortened to four seconds for grenades which were not intended to be launched from rifles. This made it almost impossible for the intended target to react to the grenade, but required diligence on the part of the quartermaster to avoid giving out rifle grenades to those not expecting them. The consequences of such a mistake would have been disastrous whatever substitution was performed.