I got this idea from a guest speaker in one of my English classes, in which she said when she is writing a story she could tell you the exact contents within her characters closet even if that never comes up within the story. This is about knowing your character inside and out.
So for the challenge write a description of the contents of the inside of your characters closest. It can be a new character you create for this, or you can use an older character created for another story. What is revealed should give the reader some basic factual information about your character. Gender, approximate age, some personality traits, etc… show how well you know your characters.