Glass bottles can be used to make Water Bottles which can be used to brew items with a Brewing Stand. Bottles may only be filled by using a water source block or a filled Cauldron. If filled via water source, the water will be unaffected. If filled via cauldron, however, 1/3 of the water in the cauldron will be removed. Therefore, using a Cauldron to fill bottles is inefficient, except in the Nether where it is normally the only way to fill bottles. Glass Bottles are used to hold potions.