Pinkie invites Sweetie Belle over for some fun. Little does Sweetie suspect that Pinkie has a different kind of fun in mind... little does Pinkie suspect that this might be harder than she assumes.
Celestia has gotten fed up of all the letters coming in, day by day-- or the citizens coming in to ask her-- about what she will and won't allow. How do you get rid of that?
When Celestia has a child, the least Equestria is worried about is who the father was, it is why her child is blind. Luna's child is star-struck and Discord's child is the opposite of her father.