Right up there with the telephone, landmark medical breakthroughs and other great innovations to come out of the Boston area is the invention of Marshmallow Fluff in Somerville’s Union Square in 1917. To celebrate that momentous turning point in human history, the aforementioned neighborhood’s annual What the Fluff? A Tribute to Union Square Innovation takes place September 21 from 2–6 p.m. The 19th edition of this whimsical street fair—which follows the theme of “Somerville Over the Rainbow: No Place Like Fluff” for 2024—features live music, Fluff-related games, food and craft vendors, and an overflow of colorful characters to delight both children and the child-like.  Get yourself and your family in the happiest of sticky situations by indulging in this delightful and nostalgic tradition that has been going strong for nearly two decades.