Answer by iggy

getting that random number is like this: x=Random.Range(-4, 4); http://unity3d.com/support/documentation/ScriptReference/Random.Range.html but if you want sphere to move from 4 to -4 in ping pong motion use this: transform.position = Vector3(Mathf.PingPong(Time.time, 8)-4, transform.position.y, transform.position.z);