Smoke signals

Smoke signals is an early form of the optical telegraph, developed by the native North Americans and Chinese (as in the towers of the Great Wall.) By covering an open fire with a blanket, and suddenly removing it for a short time, a puff of smoke is generated. With some training, the sizes, shapes of, and intervals between these puffs can be controlled in a way that can be observed from a long distance, and used to carry information.