We are all familiar with the feel of hunger. But what is hunger? How does it influence our appetite to food? A recent study led by Dr. Suewei Lin at Institute of Molecular Biology in Academia Sinica gives some clues to these important and fascinating questions. Using the fruit fly as a model system, Dr. Chang-Hui Tsao, a postdoc in the Lin lab and the lead author of this study, reveals a detailed neural circuit mechanism that hunger and satiety signals use to switch on or off food-seeking behavior. Similar neural circuit modules might also control hunger-driven behavior in other animals including human.