Shrapnel

Shrapnel is the term loosely used to describe splinters from high-explosive shells. The fragmentation effect of high-explosive anti-aircraft ammunition was generally what caused damage to enemy aircraft rather than a direct hit by a single round.
The tin helmets carried and worn by civil defence personnel throughout the war were mainly for protection from these sharp, jagged remainders of spent anti-aircraft ammunition which were often still hot as they fell onto the people, houses and streets below.
In those days, most boys avidly gathered shrapnel, spent bullets and cartridge casings.