[as they approach Fiona's castle, Donkey smells the air]
Donkey: Woah, Shrek, did you do that? Man, you gotta warn somebody before you just crack one off, my mouth was open and everything!
Shrek: Believe me, Donkey, if it was me, you'd be dead!
[sniffs]
Shrek: It's brimstone. We must be getting close.
Donkey: Yeah, right, brimstone. Don't be talking about "it's the brimstone". I know what I smelled and it wasn't no brimstone and it didn't come off no stone neither.