Don't be bashful! If you want comments, give comments. You don't need to be an expert photographer to express your opinion about the work of others. You don't need to go into great depth. A simple remark about cropping, composition, color, subject matter, etc. can be of just as much help to a budding photographer as an in-depth critique.
Take a photograph of a person or object with the light coming from behind the subject. Remember that this is tricky because you can get lens flare. Use what you learned in this chapter about metering for this kind of situation and take several photographs. Upload the best one and tell everyone how you compensated for the two different light conditions.