敲門磚（敲門磚|敲門甎）　

◎ 敲門磚 qiāoménzhuān

[a brick picked up to knock on the door and thrown away when it has served its purpose—a stepping-stone to success] 揀磚頭敲門,門開后即棄磚。比喻謀求名利的手段

這是敲門磚,敲開便丟下他。我們既作了官,做詩何用。——明· 西湖居士《春游》