TikTok would like to sell me a brush. Or, more specifically, an UNbrush. The $18 implement, made by the hair-tool brand FHI Heat, looks like a regular paddle brush, but with its rear panel removed so that air can flow through its perforated sheet of plastic bristles. Its promise, according to the dozens of video reviews that TikTok has pushed into my feed in the past week, is simple: The UNbrush cuts through tangles like a hot knife through butter, even if your hair is highly textured or coated in salt water after a day at the beach. At times, every third or fourth video in my feed has shown the brush doing exactly that, accompanied by a coupon code—never the same one, never for the same amount—and a link to buy. The UNbrush is just one of many assorted products—mascaras, office chairs, battery-powered kitchen scrubbers—that have recently gone viral on TikTok Shop, which officially launched in the United States last month. TikTok’s endless stream of unpredictable, algorithmically selected clips has long been a powerful—if …