By following giant arrows planted in the ground! In 1924, the Department of Commerce installed 1,500 illuminated, concrete, arrow-shaped beacons across the country to help pilots deliver the mail. The arrows were painted bright yellow, with a 50-foot beacon tower on top. Each arrow pointed to another beacon, installed about 10 miles away. It sure beat the old system: Before the arrows, pilots simply had to keep an eye out for bonfires lit next to landing strips.