In the Rainbow of Dog' SUDOKU Expert, available in Medium size, the movable tiles are made of polypropylene, which has been certified as suitable for contact with edibles. We made this version of the game at request of these pet parents whose dogs produce sizable amounts of saliva, while playing with the puzzles. Please make sure that you pet never plays a game unsupervised since it is an interactive puzzle designed for sharing quality time between the human and his dog.