蛙今天新學到的：
bang/bash/beat/hit/knock one's head against a brick wall
白費力氣，枉費心機；試圖幹不可能成功的事

從蛙高中畢業陪伴至今的朗文當代高級英英‧英漢辭典(都陪蛙拿到碩士了說，大推！)裏面看到很有意思的例句：
Trying to get that class to learn anything is like banging your head against a brick wall!
 要想讓那個班的學生學點東西簡直是白費力氣！
這例句的口氣聽來像極了「極道鮮師」漫畫裏的臺詞，而且還是從漫畫裏的教導主任口中說出的呢！

別拿腦袋往牆上撞，Don't bang your head against a brick wall!  比喻別試圖去幹不可能成功的事情。