Why does calling Enumerable.First() appear to return a copy of the first item in the enumeration
public class Thing
    {
        public bool Updated { get; set; }

        public string Name { get; private set; }

        public Thing(string name)
        {
            Name = name;
        }

        public override string ToString()
        {
            return string.Format("{0} updated {1} {2}", Name, Updated, GetHashCode());
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("IEnumerable<Thing>");
            var enumerableThings = GetThings();
            var firstThing = enumerableThings.First();
            firstThing.Updated = true;
            Console.WriteLine("Updated {0}", firstThing);
            foreach (var t in enumerableThings)
                Console.WriteLine(t);

            Console.WriteLine("IList<Thing>");
            var thingList = GetThings().ToList();
            var thing1 = thingList.First();
            thing1.Updated = true;
            Console.WriteLine("Updated {0}", thing1);
            foreach (var t in thingList)
                Console.WriteLine(t);

            Console.ReadLine();
        }

        private static IEnumerable<Thing> GetThings()
        {
            for (int i = 1; i <= 3; i++)
            {
                yield return new Thing(string.Format("thing {0}", i));
            }
        }
    }
}

IEnumerable<Thing>
Updated thing 1 updated True 37121646
thing 1 updated False 45592480
thing 2 updated False 57352375
thing 3 updated False 2637164
IList<Thing>
Updated thing 1 updated True 41014879
thing 1 updated True 41014879
thing 2 updated False 3888474
thing 3 updated False 25209742

IEnumerable<Thing>
Updated thing 1 updated True 45592480
thing 1 updated False 45592480
thing 2 updated False 57352375
thing 3 updated False 2637164
IList<Thing>
Updated thing 1 updated True 41014879
thing 1 updated True 41014879
thing 2 updated False 3888474
thing 3 updated False 25209742