Invented in Somerville’s Union Square in 1917, the New England culinary staple known as Marshmallow Fluff comes full circle yet again when the 12th annual What the Fluff? festival takes over the place of its birth on September 23 from 3–7 p.m. This year’s theme, “FluffCentennial: Inspiring the Next Century of Union Square Invention,” serves as a launching pad for all kinds of sticky, sweet fun, including a cooking contest (yes, Fluff can be used in something besides a fluffernutter sandwich), games, live music, craft vendors, rides and plenty of other family-friendly activities for the kids, ensuring a truly sweet time can be had by all.