敲门砖（敲門磚|敲門甎）　

◎ 敲门砖 qiāoménzhuān

[a brick picked up to knock on the door and thrown away when it has served its purpose—a stepping-stone to success] 拣砖头敲门,门开后即弃砖。比喻谋求名利的手段

这是敲门砖,敲开便丢下他。我们既作了官,做诗何用。——明· 西湖居士《春游》