Newton's Cradle Balance Balls NIB - Every action has an equal but opposite reaction-right before your very eyes - Science demonstrated clearly and simply and with a touch of amusement - To play with the balance ball is a game of fun. If you pull up one ball and release、 you will see only one ball move from that reaction. If you pull up two balls、 two balls will react. Dimensions: 5.31 in x 4.49 in x 5.51 in (13.5 cm x 11.4 cm x 14.0 cm) Weight: 6.77 oz (192 g)