I have something to tell you.

Yeah?

I, uh, well, I used to be human, but I'm not anymore. Not quite.

Oh. Well, I'm human, but wasn't always. I was something else. You're still a person - you're still Sam.

So, uh, we're good?

Yeah.

... So, what were you?

...

Sorry. Curious.

I was a magical key.

A key?

Yeah.

A magical key?

Yes, a magical key.

... To what?

...

Never mind.

Just a short little Sam/Dawn conversation. Sam's referring to the fact that he has demon blood (probably before the whole vessel arc).