Cauldron

A Cauldron is a block that can be filled with water when right-clicked upon with a Water Bucket. When filled, a Cauldron can be used to fill three Glass Bottles. These Water Bottles are key ingredients for the Brewing of Potions using a Brewing Stand.

History
Cauldrons were originally intended to be used for the creation of Potions. The system was implemented in Beta 1.9 Pre-Release 2 with Glass Bottles being obtainable, but Cauldrons not being assigned with a block ID, hence making Potions unobtainable. Cauldrons could be made to work with a Mod, where the following was revealed:
 * Cauldrons could be filled with a water bucket.
 * Applying a potion ingredient to the water would cause it to change to a differently-colored lava texture, and adding any further potion ingredients to the filled cauldron would cause the liquid to change color.
 * Scooping out a potion with ingredients applied would cause the potion to have an effect related to the ingredient, e.g. a potion brewed with a Magma Cream would have Fire Resistance as an effect.

This system was complex and not user-friendly, and hence the Cauldron's ability to brew potions was replaced with the Brewing Stand in Beta 1.9 Pre-Release 3. Cauldrons were given a Block ID and Item ID and made craftable, but all ability to apply potion ingredients to a Cauldron filled with water was removed. This meant that a Cauldron's only functional purpose was to fill three Glass Bottles with water, which made it very useful when Brewing potions in the Nether.

Trivia

 * Cauldrons cannot hold Lava or Milk, but the potions that were once put inside of it in Beta 1.9 Pre-Release 2 used the Lava texture.
 * When broken, Cauldrons do not drop any water, only the Cauldron itself.
 * When a Cauldron is being mined with water in it, the water will start to get cracks in it as if it is a block.
 * As of 1.0.0, Cauldrons can be used to legitimately store water within the Nether, as opposed to water evaporating by trying to place it down with a bucket.
 * Even if a Cauldron has water in it, a Piston can push it.
 * Even if a Cauldron has water in it, it will not extinguish any living entities (mobs or players) that are on fire.
 * In creative mode, if you attempt to fly inside a cauldron with a block above your head, a glitch will occur causing you to be "stuck" in flight form without being able to lift or lower yourself.
 * Standing in a Cauldron while the cauldron is being pulled by a piston will cause you to go through the cauldron rather than to be carried with it.
 * On November 8, 2011, Jeb tweeted a reply to a suggestion for boiling food in a cauldron, saying he may consider it later.
 * It's possible to push a minecart inside of a cauldron by using pistons and slabs.
 * It is actually extremely inefficient to use cauldrons as a source to get water for potions since 1 source block of water is enough to produce an infinite number of water bottles, crafting a cauldron requires 4 more iron ingots than it takes to craft a bucket and a filled cauldron will only fill 3 bottles before it needs to be refilled itself. Thus, the cauldron's only actual purpose is for making potions in the Nether.
 * Fire goes out shortly after putting a Cauldron over it.
 * There is a glitch when you put water in a Cauldron then use 2 glass bottles to collect water then remove the water by a bucket and replace it it goes back to the "full" state.
 * There is a glitch where if you use a Cauldron to fill an empty potion in the Nether, the potion won't appear in your inventory until you open a chest.
 * You cannot see items dropped inside a Cauldon while it's filled with water.
 * In single player in 1.9 pre release, you cannot craft cauldrons.