Computer science, to be precise. Tugs, each pulling four loaded cargo carts, stop at an unloading station. Workers empty the containers, positioning each package so scanners can read the bar code. The sorting process is three tasks occurring at once. The letter packages, which get shipped in large plastic bags, are fed by hand onto a conveyor belt.
Scanners record their destinations and then place each package onto an individual tray.
The tray moves along to a series of chutes, each chute representing a city such as Phoenix or Orlando. When the letter reaches that chute, a computer tilts the tray and dumps the letter. It then falls into another plastic bag with similarly routed letters. Boxes are treated similarly. As they move along the black bands of the conveyor belts, scanners read their destinations.