All dogs have an amazing sense of smell, much more advanced than that of a human. That sense of smell was improved in certain breeds, and these breeds were used to "track" game or people; tracking involves following a scent trail left by a person or animal and a dog's ability to track has proven to be useful to humans on many occasions. Dogs have been used to track missing persons and they have been used to track prey on hunts. The Harrier is a breed of dog that was developed as a scent hound, with an excellent sense of smell. While Harriers are not used on the hunt as often as they once were, you can still put that nose to good use and have your dog participate in tracking activities and games.
There are a number of organizations, including some of the major kennel clubs, that sponsor tracking events for dogs. As with agility, the majority of these tracking trials have no specific requirements regarding the breed or appearance of a dog that is allowed to participate. Many of the organized tracking events offer titles and prizes for dogs who excel at the sport, though the majority of dog owners participate in order to bond with their dogs and give their dogs an outlet in which to use one of their natural abilities. It is also an amazing experience to watch a dog track a scent; a world invisible to humans is clear as day for dogs.
Harriers do especially well in tracking trials, given their history; besides being ABLE to track, though, they LOVE to track and this love is obvious every time they put their nose to the ground. They are also very trainable dogs and so can perform any trail-related command required of them. Their endless energy allows them to tirelessly follow a trail until whatever is lost has been found. Harriers most often follow the ground trail of intended prey or a lost person; other dogs, however, may keep their noses off the ground more often and follow scent that wafts through the air.
Though Harriers usually don't show any resistance in their desire to track, there are some scent games that you might want to play with your Harrier puppy before you start having him participate in organized tracking trials, to prime him. A basic scent game that owners often inadvertently play with their dogs is "Which hand?". Place either food or a small toy (that your dog likes) in one hand and then present both hands, closed, to your dog; ask "which hand?" and only give the dog the treat if he correctly chooses the right hand. It's up to you to decide how you want your dog to "choose" your hand. You can also play a game with your Harrier where you show the dog a treat and then hide it in a dark room; give him a command and allow him to enter the room in search of the object. Shower him with praise when he is successful. Other games include hide and seek, where you, the handler, do the hiding and the dog does the seeking, and dropping or throwing objects that your dog must find.