This product makes it easy to judge if you're throwing correctly by sight and sound, allowing for fun and engaging play. * A whistling sound ("Hyu") is produced when thrown straight. * The ball wobbles if the direction or angle of the throw is incorrect, making it easier to focus on proper throwing technique. Javelin Ball Throw Training Ball for Introductory Period Javelin Ball Mini, designed for the physique of elementary school students and lower/middle grades. Size: Approximately (diameter) 78 x (L) 288mm Weight: Approximately 125g Materials: (Ball) Foamed polyurethane, (Whistle) ABS, (Joint) EVA, (Wings) LDPE I purchased this but have not used it and have been storing it. There are no noticeable stains or scratches, but as this is a used item, please refrain from purchasing if you are concerned or are a perfectionist.