Mrs. Black lives in a small village. Her husband is dead. Her son, Jack, lived with her. He worked in a shop in the village, but then he found a job in town named Green Sea. He went to live there. Mrs. Black was very angry. She got on a train and went to see her son. Then she said to him, "Jack, why do you never telephone me?"
Jack laughed, "But, mother, you haven’t got a telephone. "No." She answered, "I haven’t, but you have got one. "

若$\theta$是$△ABC$的一个内角，且$\sin \theta \cos \theta =-\frac{1}{8}$，则$\sin \theta -\cos \theta$的值为(    )