Lena had always laughed when her dog, Milo, sniffed her whenever she came home. He usually circled around her legs, investigated her clothes, and then settled happily beside her. But one afternoon, something changed. Milo ignored his favorite toy and walked straight toward Lena, carefully sniffing around her. He seemed unusually focused, moving from her jacket to her hands and then back again. Lena smiled and tried to distract him, assuming he had simply caught an unfamiliar scent. After all, dogs experience the world through their noses. Every person carries countless chemical scents that can change because of exercise, stress, food, hormones, or the environment. To Milo, Lena was practically a walking collection of information.

Then Milo suddenly stopped, stared at her, and began scratching urgently at the pocket of her jacket.