The preposition OF rules

Question

She climbed out of the swimming pool. The ball flew out the window. He went out the door.

Why do we use the preposition OF in the first sentence, but not in the last two sentences.

Answer 1

Use is not entirely predictable - there are historical and dialectal variants - but generally

• We use out of when the object of the compound preposition is the structure or area within which the subject is initially enclosed and which he departs:

  She climbed out of the swimming pool. = She left the swimming pool.
  He went out of the shop into the street. = He left the shop.

• We use out when the object of the preposition is the opening through which the subject passes:

  The ball flew out the window. = The ball flew out (of the house) through the window.
  He went out the door into the street. = He went out (of the shop) through the door.