On the desert planet bottletopia (great name, I know), the art of bottle-making, and by extension, glass-making, is a closely guarded secret, known only to the elite members of society. The world is covered in one solid desert with major cities being located at the few large oases. It is impossible to travel between the cities unless you have a bottle. For unknown magical reasons, any liquid that is stored in any non-glass container disappears when it is brought far enough into the desert. Because glass is made from the desert sand (this is the secret that must be kept) the desert seems to ignore water inside it, as it sees the glass as still being sand. You basically have to smuggle water through the desert in order to survive.
Shoes.