The idiom a penny for your thoughts is a way to ask people what they are thinking about. You'd ask this if someone has been sitting looking quiet, or like they are daydreaming.
This dates back to the 1500s, when a penny was worth a lot more than it is now.
This dates back to the 1500s, when a penny was worth a lot more than it is now.
You've been staring out the window for half an hour now. A penny for your thoughts?