In Act 1, Scene 5, how do Romeo and Juliet find out each other's last names?
This scene is the one where the Capulets have thrown a big party at their home -- a masked ball. Romeo and his friends have come uninvited and Romeo has met Juliet. They have fallen in love right away, each not knowing who the other is.
The nurse (Juliet's nurse) tells each of them who the other is. She tells Romeo because she comes over and tells Juliet that her mother wants to see her. Romeo asks who Juliet's mother is and the nurse tells him. She tells Juliet just because Juliet asks her who Romeo is.