We started talking about the circulatory system and created edible models of the heart ("blue" icing depicts deoxygenated blood and "red" icing depicts oxygenated blood). The crackers each represent one of the four chambers (top are the atria and bottom are the ventricles) and the marshmallows represent (crudely!) the blood vessels. Up next? Dissecting a cow's heart!
Of course we also played with our models and then ate them!