Pokemon Sword and Shield has the most British problem ever: its teapot and teacup market is flooded with counterfits and that's. Earlier this year, Pokemon Sword and Shield introduced Polteageist - a Ghost-type Pokemon that lives in a teapot. The Pokemon seems to be made up of a combination of residual spiritual energies and tea residue and is probably the most delightfully British Pokemon in a game that wears its British influences on its sleeves. Now that the game is out, we know that Polteageist is the evolved form of Sinistea, another Ghost-type Pokemon that lives in a teacup.
Players need to obtain either a Chipped Pot or a Cracked Pot and use it on Sinistea to evolve it into Polteageist. While the two pots act as standard evolutionary items (similar to an Evolutionary Stone), each pot is used to evolve a different form of Sinistea. Most Sinistea found in the wild live in a forgery of an apparently rare kind of teacup. The common Forgery Form of Sinistea uses the Cracked Pot to evolve. However, 1% of Sinistea live in genuine teacups and evolve using the Chipped Pot. While players can't tell the difference between the Forgery Form of Sinistea and the Non-Forgery Form of Sinistea, the Non-Forgery form of Polteageist has an actual stamp of authenticity that can be found on the bottom of its teapot.
As of right now, there doesn't seem to be any difference between the two forms, other than their rarity. If you discover a Sinistea that doesn't evolve using the Cracked Pot, you'll need to track down one of the Chipped Pots in order to complete the evolution.