Presenter notes: Open with a question that sounds unpleasant and is actually the whole topic. Ask what would happen to a person who ate normally, drank normally and breathed normally, but could not get rid of anything at all. Wait, because the class will look for something that is missing and there is nothing missing. That is the point. Excretion is the only life process where failure means having too much rather than too little, and a person is poisoned by material their own body made. Tell them that today they will find out why a plant needs no kidney, and that the answer is not because plants are simple.