One of the central questions in condensed matter physics is how electrons move in solids, giving rise to the familiar distinction between metals and insulators. In metals, electrons spread out and move freely, while in insulators they are somehow prevented from carrying current. This can happen in two different ways: either because all the available energy states are already filled (a band insulator), or because strong repulsion between electrons forces them to stay put (a Mott insulator). In this talk, I will introduce these ideas with simple models and then turn to the case of 1T-TaS₂, a material that has long been regarded as a prototypical Mott insulator. Our recent theoretical work shows a more subtle picture: a single atomic layer of 1T-TaS₂ does behave as a Mott insulator, but the bulk crystal is actually insulating for a different reason, linked to the way the layers interact. This new perspective helps make sense of several puzzling experimental observations and provides a fresh understanding of this classic material.