An interface should feel at home on a tiny phone, a tablet, or a wide desktop screen—without ever making you stretch your thumb or squint. And when things don’t go as planned, a well‑designed interface doesn’t just display an error; it guides you gently back on track. In this chapter we’ll look at the details that make a design adaptable: adjusting to different screen sizes, and handling those quiet moments when something is missing, broken, or waiting for a decision.