Fairy tales are full of impossible tasks:
Gather the chin hairs of a man-eating goat,
Or cross a sulphuric lake in a leaky boat,
Select the prince from a row of identical masks,
Tiptoe up to a dragon where it basks
And snatch its bone; count dust specks, mote by mote,
Or learn the phone directory by rote.
Always it's impossible what someone asks—

You have to fight magic with magic. You have to believe
That you have something impossible up your sleeve,
The language of snakes, perhaps, an invisible cloak,
An army of ants at your beck, or a lethal joke,
The will to do whatever must be done:
Marry a monster. Hand over your firstborn son.