Whenever we think about mood we are actually talking about the feeling or atmosphere that a given story evokes. Thus a horror story could evoke a mood of fear and trepidation, or a story about nature in the countryside could evoke a mood of peace and tranquility. The important thing is to consider what happens and then to think about what emotional feeling it has on you. When we consider Act III scene 1 of this excellent tragedy, we can see that there are two distinct moods. There is one of lighthearted humour that accompanies the banter between Mercutio and Romeo as they engage in their verbal battle of wits, and then there is a much darker, suspenseful mood as Tybalt kills Mercutio and is then killed by Romeo, concluding in the Prince's exiling of Romeo:
Let Romeo hence in haste,
Else, when he is found, that hour is his last.
Bear hence this body and attend our will.
Mercy but murders, pardoning those that kill.
Note the way in which Romeo's grief and rage against his situation is captured in his line, "O, I am fortune's fool!" The mood has changed distinctly from the easy, light-hearted one at the beginning of the scene to one of tragedy, as Romeo has been forced to kill Juliet's kinsman, who is actually now his kinsman now that he has married Juliet.