Learning is Fun! Newton's Cradle Balance Balls are a perfect way for kids to learn about the conservation of momentum & energy. They are great for psychology, science, and physics subject discussion. Each cradle has a dimension of 4.6" x 1.5" X 3.7". This Newton’s Cradle toy is also a great present for decorating an office. It is an effective way to relieve stress and frustration for working people and children. Sold by the individual item. Please order in increments of 1.