Smoke signals were utilized by American Indians to warn others of a variety of situations, including danger, calling people to a common meeting spot, and transmitting news. This old talent is part of a larger category of nonverbal communication among American Indians. For American Indian communication, various sorts of communications were necessary, the most important of which was sign language. Because each tribe had its own language, these nonverbal cues allowed people from different tribes to communicate with one another. This kind of message transmission was also very useful in mountainous or highly forested areas. Smoke signals were adopted by people all around the world, not just Native Americans. Smoke signals were used by Chinese soldiers stationed along the Great Wall of China to communicate messages of approaching attack from one watchtower to the next. To make dense smoke that can be seen from afar, they employed a mixture of wolf excrement, saltpeter, and sulphur. They were able to send a message as far as 300 miles in just a few hours by transmitting the message from tower to tower.