My script wont work!

Im trying to do a simple script but it keeps giving me this “no extension method tag' of type UnityEngine.Collision’ could be found (are you missing a using directive or an assembly reference?)”
Here is my script
using UnityEngine;
using System.Collections;

public class Cube : MonoBehaviour {

int health = 100;

int sphereDmg = 20;

	void OnCollisionEnter(Collision other) {
		if (other.tag ("Sphere")) {
		health = health - sphereDmg;
		Debug.Log (health);

please help


(other.CompareTag ("Sphere")

Instead of

(other.tag ("Sphere")) {