The Nature First Willow Ball is an ideal treat and toy for rodents. The ball is 100% natural, made from willow, it is healthy, safe, keeps their teeth clean and sharpens them. For endless hours of fun and exercise. It contains low levels of salicin (a substance found in the bark of white willow, with anti-inflammatory action, acts as a natural antipyretic and analgesic, the main substance of aspirin).