In this activity, learners investigate the olfactory system by conducting several experiments. In "CLASS EXPERIMENT," learners find that the ability to detect an odor decreases with continued exposure to that odor, a property called olfactory fatigue. They learn basic facts about sensory receptors, nerve connections, and brain areas, and discover what conditions can affect olfactory fatigue. In "TRY YOUR OWN EXPERIMENT," learners design experiments to further explore the sense of smell. For example, they can investigate whether mixing two substances makes it harder to identify odors; whether gender affects the ability to identify odors, and whether people vary in their ability to match unlabeled containers of odor materials. This lesson plan includes background information, tips for educators and separate guides for educators and learners.