A penny for your thoughts. also A penny for them.
The saying “a penny for your thoughts” is an English idiom simply asking people to volunteer their opinions on an issue being discussed. Though no payment actually changes hands, the phrase has become a regular part of the English vernacular. In modern usage, it is often stated as an indirect way of asking what someone is thinking about or what is bothering them. Its origins are fairly unknown though it dates back to at least the end of the Middle Ages.
This phrase is basically a proposal, and the speaker is offering to pay to hear the listener’s thoughts. It is an idiom, of course, and not meant literally so no real payment generally takes place. The idea, however, is that the person who says “a penny for your thoughts,” wants to know what the listener is thinking about and is showing interest through a symbolic offer of payment. It is also commonly used when someone seems to be deep in thought or troubled by an idea, as a polite way of giving the person an opportunity to express his or her ideas or concerns.
I would use this as a Plenary for the end of a lesson or even a unit. I would teach what this meant and why it is used in the world of English. Personally; I am always using this but not as much as I thought I did. I have found that people older them myself (a lady never tells her age) ask me (and others my age and below) “a penny for your thoughts” more.