What Tristan's teacher wants the students to avoid in life but pursue in their fiction is irony.
<h3>What is irony?</h3>
Irony is a statement that when taken in context actually mean something different from or the opposite of what is written literally. It is the use of words expressing something other than their literal intention, often in a humorous context.
Therefore, Tristan's teacher is encouraging his students to use irony when writing fiction but they should avoid it as much as possible when dealing with real life situation.
