“敲门砖”，字面意思指“拣来敲门、门开后即被丢弃的砖头（brick picked up to knock on the door and thrown away when it has served its purpose）”。常用来比喻世人借以猎取功名的工具（thing used as a means for fame and position），一达目的，即可抛弃。

例句：

有些人常把读书当成“敲门砖”，一旦功名到手，书籍也就被束之高阁了。

Some people take attending to school only as a stepping stone to status; once they succeed, all the books will be shelved.