Minecraft wiki entry for Potion, associated text: Using a glass bottle on a water source block or a cauldron that has water turns it into a water bottle. Using a glass bottle on a cauldron that contains a potion turns it into a bottle of that potion.‌[Bedrock Edition only] Using a glass bottle on a cauldron also removes 1⁄3 of that cauldron's contents, but using a glass bottle on a water source block does not affect the water. Filling bottles from a water source (but not a cauldron) can be automated by placing the bottle(s) in a dispenser.