口

How it works

を (object) marks 口(くち) (mouth) as the thing being acted on, and the verb 開ける(あける) (open) appears as the te-form 開けて(あけて) (open). On its own, a te-form like this works as a soft, casual instruction — “open your mouth” — gentler than a blunt command. It also leaves the sentence feeling unfinished, as if more could follow.