IDIOMS

penny for your thoughts

(idiomatic) Used to inquire into the thoughts and feelings of another, especially when the person appears pensive or conflicted.

Example: 1749, Henry Fielding, The History of Tom Jones, a Foundling, Book IV, chapter xiv
  One day, as your ladyship was playing on the harpsichord to my master, Mr Jones was sitting in the next room, and methought he looked melancholy. La! says I, Mr Jones, what's the matter? a penny for your thoughts, says I.