A small number of people employ guide dogs to assist in mobility. These dogs are trained to navigate around various obstacles, and to indicate when it becomes necessary to go up or down a step. However, the helpfulness of guide dogs is limited by the inability of dogs to understand complex directions. The human half of the guide dog team does the directing, based upon skills acquired through previous mobility training. In this sense, the handler might be likened to an aircraft's navigator, who must know how to get from one place to another, and the dog to the pilot, who gets them there safely.