rack one's brains 苦思冥想

“苦思冥想”，汉语成语，比喻绞尽脑汁，深沉地思索。可以翻译为“rack one's brains（to evolve an idea），think hard and long或ponder over something all day long”等。

例句：

他苦思冥想，还是不得其解。

He failed to find an answer after pondering long and hard over the question.