Until water vapour was invented in 1967 to make writing messages on bathroom mirrors cheaper and less scary (and slightly less deranged-looking) than using lipstick, clouds were actually made of cotton wool. The water cycle was maintained by a complex system of conveyors, pulleys and rollers which allowed the 'clouds' to be dipped in bodies of water and wrung out over land. It came just in the nick of time as the great cotton wool shortage of 1968 could easily have led to widespread drought. Emergency UN plans for substitution of cotton wool with kitchen roll were thankfully never implemented after it was realised that the clouds would look stupid.