Cleric Spells, How do they get them?

There was an interesting quesiton brought up in the osrgaming.org forums about Cleric spells. The question asked was

Do they (Clerics) automatically know all spells for the level they are, but only can cast what the memorize? Or are they like magic users and have to copy from scrolls or the like?

Interesting question. Now, we all know from playing any edition of D&D that Magic Users, study a book, or research a spell. Then they memorize the spell for the day to use it, and once they do, it is cleared of their mind and the process starts over again.

But what about Clerics?

They pray for their spells is the answer everyone gives.

But how do they actually know what spell is what? How do they know what to say and when to say it?

I have always thought that a Cleric, would pray for that spell, and it would just "pop" into their mind.

That was good enough reason for me, up until I saw the post. Then it had me thinking, how does it really get there, and how does a Cleric know the words.

Faith.

Plain and simple, the cleric prays for the "will" to change things and in return their god grants them a small amount of power to change the world or the things around them in the world.

So with that said, I'd like to hear what everyone else thinks.