Dogs are trained to sniff out drugs by playing games. First, they just get to have fun playing tug-of-war with a towel. As time goes on, marijuana is hidden in the rolled-up towel and the dogs learn to associate the smell of marijuana with their favorite tug-of-war game. Later, the trainer hides the towel and drugs, and the dogs sniff around to find them. This way, the dogs learn that if they find drugs, they'll get to play tug-of-war. The same method is used to teach police dogs to sniff out other drugs and even explosives.
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