Berry Juice is a HP restoring item introduced in Generation II. It is produced by giving a Berry (in Generation III an Oran Berry) to a Shuckle to hold. According to Shuckle's Pokédex entries, the berries it stores in its vase-like shell decompose and become a gooey liquid that is Berry Juice. Berry Juice restores 20 HP.
In Generation II If the Berry Juice is held by a Shuckle, it will, over time, harden into a Rare Candy. This has been removed for use in later games.