Shoot only once when clicked

im trying to get a single projectile to be fired every time i click the mouse button. but at the moment i am only getting a continous flow of projectiles. any ideas?

var projectile : Rigidbody; var speed = 1; function Update () { if(Input.GetButton("Fire1")); { clone = Instantiate(projectile, transform.position, transform.rotation); clone.velocity = transform.TransformDirection( Vector3 (0, 0, speed)); Destroy (clone.gameObject, 3); } }