At first glance, a tight-fitting, knee-length LBD with cold-shoulder details may not seem so functional. But Katie Holmes, who, let us remind you, rode the train in a ballgown, was up for some action when she visited Jimmy Fallon on The Tonight Show. The actress must have felt supremely comfortable in her good friend Zac Posen's design, because she mimicked Beyoncé's halftime dance and jumped across the table during a game of Musical Beers with Ryan Reynolds. 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.