Rigidbody not colliding...? or is my code wrong?

So I am making a top-down shooter, and I have a shell ejection system. I want to have a little shell tinkling sound play every time a shell collides with something. I have a rigidbody on the shell (so it can collide and bounce around, of course) and I have a script that should be playing the sound on collision. But it doesn't. Any idea why?

  void OnCollisionEnter (Collision collision)
         {
                 print("Object has collided.");
                 print("Sound successfully played.");
                 Shells.Play();
         }

It doesn't even print in the debug log. So I'm assuming I did something wrong. Thanks in advance!