Objects in my scene are not responding to OnCollisionEnter

On an object im my scene I have the code:

function OnCollisionEnter(collision : Collision) { Debug.Log("hit"); for (var contact : ContactPoint in collision.contacts) { Debug.Log(contact.thisCollider.name + " hit " + contact.otherCollider.name); } }

However this is not working and the objects collide and stop when they hit each other but they do not send the debuging message. Does anyone know how to fix this or what I am doing wrong?