At first glance, a tight-fitting, knee-length LBD with cold-shoulder details may not seem so functional. But Katie Holmes [1], who, let us remind you, rode the train in a ballgown [2], was up for some action when she visited Jimmy Fallon [3] on The Tonight Show. The actress must have felt supremely comfortable in her good friend Zac Posen [4]'s design, because she mimicked Beyoncé's halftime dance [5] and jumped across the table during a game of Musical Beers [6] with Ryan Reynolds [7]. Ladies, you might as well add this simple-yet-striking sheath to your wardrobe right now, because we doubt you'll ever find a number so movable yet structured and appropriate. Read on to watch Katie work it, then shop similar dresses.