it should open in a small way and only open all the way with help. If you are talking about child birth that is different the vagina will open by the pelvic area pushing to the sides and making the birth canal.
Pinching the bottom end of a banana and peeling from there is the most foolproof way to open it, but many people choose to try to open from the top, which doesn't always work.
It's not closed. If you mean the hymen it can break from doing sports, using fingers when masturbating or many other things and many is born without one.
Many places have a law that requires doors on PUBLIC buildings to open outward so that in the event of an emergency you cannot be pressed up against the door by a crowd and unable to open the door.