Every board hides groups of four tiles that share a category.
Drag a tile onto another to swap them, or tap one tile and then the other.
Line up all four tiles of a group in one row and it locks in, revealing the category.
Or use the potion vials: tap a vial, then ink four tiles. If all four share a category, they lock in without lining them up.
Each vial colors one group and is then used up. Groups you solve by lining up take your ink color if you marked them, otherwise the next unused vial from the left.