Wacky C#: yield return

Right off from the canonical "C# Language Specification".

static IEnumerable Range(int from, int to) {
for (int i = from; i < to; i++) {
yield return i;
}
yield break;
}
static void Main() {
foreach (int x in Range(-10,10)) {
Console.WriteLine(x);
}
}

What the heck is "yield return"? The purity of return statement is whacked. It seems each yield return is for each of the items in that IEnumerable.

I saw this example too, in another place, where Fib is of course the (fabulous) Fibonacci numbers:

public static IEnumerable Fib(int a, int b)
{
yield return 1;
yield return 1;
yield return 2;
yield return 3;
yield return 5;

}

static void Main()
{
foreach (int i in Fib(1, 5))
{
Console.WriteLine("{0} ", i);
}
}

I'll list more wackiness when I see them