Difficult is not an absolute. It needs to be "in comparison to something". Desalination is verrrrry easy if nature does it for us (sun on the salty sea -> clou

Difficult is not an absolute. It needs to be "in comparison to something". Desalination is verrrrry easy if nature does it for us (sun on the salty sea -> clouds -> precipitation over land). Compared to this, doing it "with a machine" is hard.