Гигель не нападает на жизнь деревьев

        Sprite gigel = new Sprite(WindowsFormsApplication2.Properties.Resources.Gigel_img);
        Sprite tree = new Sprite(WindowsFormsApplication2.Properties.Resources.tree_img);

        void Screen_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.E)
            {
                gigel.Atack(tree.life);
                tree.LifeUpdate();
            }
            gigel.Reposition();
            Refresh();
        }

...
inside Sprite class 
        public void Atack(int targetLife)
        {
            targetLife -= damage;
        }

//I hope now is more clear: 

            if (e.KeyCode == Keys.E)
            {
                int num = 20;
                gigel.Atack(num);
                tree.life = num; // num is 20 and not 7

                tree.LifeUpdate();
            }
...
inside Sprite class 
        public void Atack(int targetLife) //targetLife = 20
        {
            targetLife -= damage; // 20-13 targetLife=7
            //from this point (num) must take targetLife value of 7
            //but (num) remains at 20.
        }