User:大头儿子的好爸爸/sandbox

This sentence is from the article (Smoke signal).https://en.wikipedia.org/wiki/Smoke_signal?oldformat=true. The facts are from (Military communications: from ancient times to the 21st century). 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 as far away as 750 kilometres (470 mi) in just a few hours.