Buckets are tools used for carrying water, lava or milk. Water and lava buckets can only be obtained by using the bucket on a source block; shallow, running fluids cannot be collected. Milk buckets are collected by using the bucket on a cow. If one is under water and moving forward, filling an empty bucket will create a momentary air pocket. This is enough to refill the player's air meter, allowing them to stay under water indefinitely. To reuse the bucket just dump out the water.