The Clangers planet not having a thick atmosphere like our own allows meteorites and other even odder things that happen to be hurtling about in space through to it's surface. Because of this, the Clangers live inside their planet and wear armour plates. They go into their planet through holes which are protected with stout metal lids. It is the cling-clang-clonk sound of these when they are slammed shut that gives the Clangers their name.