Is your dark matter detector a bag of free particles? If you're looking for GeV-TeV WIMPS, sure! If you're looking for sub-GeV dark matter, emphatically no! I will argue that condensed matter physics is mandatory for understanding light dark matter interactions with generic solid-state detectors, and give three examples of simple models which elucidate the features required to design a quantifiably optimal detector. In particular, a perverse and accidental hierarchy between the dark matter velocity and the electron Fermi velocity in conventional materials points the way toward molecular crystals and narrow-gap heavy-electron compounds as ideal detector materials.