Imagine running a marathon, but instead of a medal at the end, you've got to deliver a message. That was the life of the Chasqui, the Inca messengers. Using a relay system across thousands of miles of roads and bridges, these runners could convey messages swiftly across the empire. They also used quipus, a system of knotted strings, for record-keeping and communication, which is pretty mind-blowing when you think about it.