BCE0005: Unknown identifier: 'spriteRenderer'.

#pragma strict

var diamond : Texture;
var square : Texture;
var triangle : Texture;
var circle : Texture;
var start : Texture;
private var number : int = Random.Range(1,4);

function Start () {
spriteRenderer.Sprite = start;
}

function Update () {
if (number == 1) {
spriteRenderer.Sprite = diamond;
}
else if (number == 2) {
spriteRenderer.Sprite = square;
}
else if (number == 3) {
spriteRenderer.Sprite = triangle;
}
else {
spriteRenderer.Sprite = circle;
}

}

I don’t get what I got wrong here. It said BCE0005: Unknown identifier: ‘spriteRenderer’. Anyone help please? Thanks

http://unity3d.com/search?refinement=answers&gq=bce0005