Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent six days a week. There are countless TikTok videos that go like this: Someone says something to their dog, the dog’s head swings to one side, with ears up and eyes on the owner. Whoever’s filming can barely hold the camera steady because it’s just too dang cute. Whether it’s online or in person, we’ve all seen dogs tilt their heads—and yet scientists don’t know why they do it.  “Dogs are singularly situated to be sentinels [mirrors] of the human experience,” Courtney Sexton, a postdoc at Virginia-Maryland College of Veterinary Medicine who studies human-dog relationships, tells Popular Science . The head tilt, she argues, is a window into one of the most striking of those adaptations. Unlike cats , hamsters , or even our closest primate relatives , dogs have developed the ability to parse human speech in ways that mirror our own.…