Sinclair Swirling is an achievement that requires the player to use the churn in the Sinclair Mansion garden.
A bucket of milk can be obtained by using a bucket on the dairy cow in the garden. Players can alternatively obtain a bucket of milk by purchasing from the milk seller for 12 coins. The churn is just north from the stall.
Sometimes, when making butter, the achievement will finish before the milk is used. Players do not have to finish making the butter if this occurs.