The Nightfury looks confused for a moment, tracking Bela's movements with the bottle, as if he can't quite wrap his head around what she's proposing. But then he recalls Hiccup's father pouring something out of a flask. It wasn't like this in shape, but perhaps the concept is generally the same.

He opens his mouth for her.

At first, the short white teeth in his bright gums are there, but after a second they retract right in. It makes him look comical, rather than vaguely menacing.