inheritance=no, interfaces=yes

gistfile1.cs

    interface IGulpable
    {
        void Gulp();
    }

    class CokeBottle : IGulpable
    {
        public decimal Volume { get; set; }

        public CokeBottle()
        {
            Volume = 355; // mL
        }

        public void Gulp()
        {
            Volume = Volume - 40;
        }
    }

    class WaterBottle : IGulpable
    {
        public decimal Volume { get; set; }

        public WaterBottle()
        {
            Volume = 750; // mL
        }

        public void Gulp()
        {
            Volume = Volume - 40;
        }
    }

    class Person
    {
        public void Drink(IGulpable beverage)
        {
            beverage.Gulp();
        }
    }