Void OnCollisionEnter() not working?

using UnityEngine;
using System.Collections;

public class PlayerHealth : MonoBehaviour {
public float RemainingHealth;

// Use this for initialization
void Start () {
	RemainingHealth = 100f;


}

// Update is called once per frame
void Update () {

}

void OnCollisionEnter(Collision coll) {
	if (coll.gameObject.tag == "Projetil")
	{
		Destroy(coll.gameObject);
	}
}

this is my health script, still working on the RemainingHealth float, but the collision doesn’t seem to be working. it doesn’t destroy the GameObject, why not?

For 2D you must use OnCollisionEnter2D instead
Hope that helps