Definition
This phrase is used to ask someone what they are thinking, often because they have been quiet or pensive.
想什么呢？（问屏沉默之人）
You've been quiet all day. A penny for your thoughts?
你一整天都很安静。能告诉我你在想什么吗？
She was staring out the window, so I asked, 'A penny for your thoughts?'
她一直盯着窗外，所以我问：『能告诉我你在想什么吗？』
You seem preoccupied. A penny for your thoughts?
你看起来心事重重。能告诉我你在想什么吗？