Objects don't collide

Hi there!

I've made a sphere which follows user's mouse. Here it's script:

 var mycamera: Camera;
 mycamera = GameObject.Find("cam").camera;
 function Update () 
 {
     var mpos = Vector3.Normalize(mycamera.ScreenToWorldPoint(Input.mousePosition) - transform.position);
     transform.Translate(mpos * ( 5.0 * Time.deltaTime ) );
     transform.position.z = 0.0;
 }
 function OnCollisionEnter(theCollision : Collision)
 {
   Debug.Log("test");
 }

I've also added a collider into it. After all, I've duplicated a sphere, so they're both following mouse.

But they don't collide (I don't see 'test' message in console).

What am I doing wrong?