The children listened open-mouthed as the teacher told an adventure story.

open-mouthed 是由形容词 open（张开的）和名词 mouth（嘴巴）加上后缀 -ed 组成的复合形容词，字面意思是"嘴巴张开的"，用来形容人因为惊讶、专注或震惊而张大嘴巴的样子。