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 were not assigned 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, due to the fact that water evaporates in the Nether when placed.

As of snapshot 12w22a, cauldrons fill with water if placed outside during a rain shower/thunderstorm. Placed next to a BUD switch, a filling Cauldron will be able to be used as a rain detector in redstone mechanisms.

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 that have water in it 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. They however are of some use if used in zones far away from water sources to collect rainwater (For example, in a Flat map).
 * Fire goes out shortly after putting a Cauldron over it.
 * 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 most items dropped inside a Cauldron while it's filled with water.
 * You cannot sneak across a cauldron, as you can still fall in.
 * Arrows "stick" to the water in a cauldron.