Animagus

I know how to pet a swan, and if you are a swan then I will pet you. You will like that.

I also know how to hold an eagle. If you are an eagle, then I will hold you. You will like that.

But if you are a swan, and as I come to pet you, you do become an eagle, then I cannot pet you. And if, as I come to hold you, you do return to a swan, then I cannot hold you.

If you are both and neither swan nor eagle, then I can neither pet you nor hold you, and you will not like that. So do not try to be both, just either.