Complex sentence: A stress ball is a malleable toy, usually not more than 7 cm in diameter, which is squeezed in the hand and manipulated by the fingers, ostensibly to relieve stress and muscle tension or to exercise the muscles of the hand.
Split into simpler sentences: A stress ball is a malleable toy, usually not more than 7 cm in diameter. It is squeezed in the hand and manipulated by the fingers, ostensibly to either help relieve stress and muscle tension or to exercise the muscles of the hand.