c# - Rotating an object continously with OnMouseDown

I have a script that works but it does not do what I want it to do. My script makes an object rotate on when another is clicked. However, I want the object to continuously rotate until I click on the other object again. Right now when I click it rotates just a bit and stops.

Here is my script:

 public class Rotate : MonoBehaviour {
    public float speed =50f;
    public GameObject target;

    public void OnMouseDown(){
       target.transform.Rotate (new Vector3(0, Time.deltaTime, 0) * speed);
    }

 }

How can I make the object rotate continuously until I click it again?