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