Invalid Cast Exception

    class Animal
    {
        public void Eat() { Console.WriteLine("Eating."); }
        public override string ToString()
        {
            return "I am an animal.";
        }
    }
    class Reptile : Animal { }
    class Mammal : Animal { }
    class Sample
    {
        public void Test()
        {
            Run(new Mammal());
        }

        void Run(Animal a)
        {
            //Reptile r = (Reptile)a;
            Reptile r = a as Reptile;
            if (r != null)
            {
                Console.WriteLine("OK");
            }
        }
    }