Object Not Recieving Damage

Ive created an Object that should act as a tower in a spacewars game, it attacks, looks at the target and aims properly and stuff, but it doesn't recieve damage for a reason, I've attached the following script on the object, whats really happening is, that the function OnCollisionEnter isnt even functioning, because the nothing is appearing on the Debug Log:

private int health=20;

void onCollisionEnter(Collision other) { if(other.gameObject.tag=="Bullet") { health-=5; Debug.Log(health.ToString(),gameObject); } }

// Use this for initialization void Start () {

}

// Update is called once per frame void Update () { if(health<=0) Destroy(gameObject);

}

}

Thanks for help.