The smoke signal is one of the oldest forms of long-distance communication. It is a form of visual communication used over long distance. In general, smoke signals are used to transmit arrival, signal danger, or gather people to a common area.
In Ancient China, soldiers stationed along the Great Wall would alert each other of impending enemy attack by signaling from tower to tower. In this way, they were able to transmit a message a very long distance.
Create a smoky fire, with damp grass, and try to send a signal to someone far away.