The term 'short-lived thing' can be summarized by two crossword solutions: ephemera (8 letters) and FAD (3 letters). Ephemera refers to objects that are transitory or quickly fading in popularity, such as postcards, tickets, or other printed materials. A FAD is a trend or behavior that is popular for a short period of time before it fades away. Both ephemera and fads are examples of short-lived things that come and go quickly.