fall between two stools Stool是凳子;在两把凳子之间犹豫，决不定坐哪一把，结果往往是两把凳子都会给别人坐了。所以，人们说"两头落空"或"两边不讨好"，会用fall between two stools(落在两把凳子之间)这个成语. If something falls between two stools, it fails to achieve either of two aims. 两头落空 例： The grammar guide falls between two stools - it's too difficult for a beginner but not detailed enough for an advanced student. 这本语法指南对初学者和水平高的学生都不适用——对初学者太难，对水平高的学生又不够详细。