Juan wants to teach his dog to bark when someone comes into the yard. At first, he rewards the dog whenever it barks. Then he rewards the dog only if it barks when a person walks by the house. Next, the dog is rewarded for barking only if a person stops by the house, then only if the person touches the gate. Finally, the dog is rewarded only for barking when a person actually opens the gate and enters the yard. Juan is using a step-by-step learning technique called.