A bloodhound is a large dog breed famous for its extraordinary sense of smell and its ability to follow a scent trail across long distances, even days after the trail was made. These dogs have wrinkled faces, long droopy ears, and loose skin that helps trap scent particles as they track. When a bloodhound puts its nose to the ground, it can follow a person's unique scent through forests, across rivers, and over miles of terrain.

Police and search-and-rescue teams use bloodhounds to find missing people or track suspects because their noses are so incredibly powerful. A bloodhound can distinguish between the scents of different people the way you might tell your friends apart by their faces. Once a bloodhound catches a scent, it follows with intense focus, pulling hard on its leash and baying (making a deep, musical howling sound) to let handlers know it has found something.

People sometimes call a determined detective or investigator a bloodhound because, like the dog, they pursue clues relentlessly until they solve the mystery.