听力原文： Sign language is a way of talking by using signs. Indians used sign language when they met strangers. In this way, they could find out whether the stranger was a friend or an enemy. In Indian sign language, signs were made with the hands. One sign meant 'man'. Another meant 'horse'. To tell the time of day when something happened, an Indian pointed to the sky. He showed where the sun had been at the time. Indians usually used signals when they wanted to send messages to someone far away. To make signals, an Indian might use a small horse. He might use smoke, a mirror or fire arrows. To signal that he had seen many animals, an Indian rode his small horse in a large circle. Sometimes the Indian gave a signal like this and then went away to hide. This meant that there was danger. He could also send many signals with a mirror. He usually used the mirror to warn someone of danger. Or he attempted to get the attention of a person far way. But he also used it to send messages in code. Of course, mirrors could only be used when the sun was shining. At night. Indians used fire arrows for signaling. In which situation Indians wouldn't use sign language according to the passage?