TRACY, Calif.— On Cyber Monday, the Amazon fulfillment center in this rural town 60 miles east of San Francisco was a whirling, zooming, bustling beehive of activity.

“Last year customers ordered 43 million items worldwide on Cyber Monday — that was 500 items per second,” said Sarah Gelman, an Amazon spokesperson.

“This year could surpass that,” she said.

Down on the floor, a steady stream of freight trucks backed into one side of the 1.1 million square-foot facility. They were unloaded by an army of stockers who opened cartons and placed individual items into shelving units called “pods.”

The elaborate logistical dance is overseen by a sophisticated computer program that knows where each item is at any given moment and makes sure that it gets where it needs to go. Because of that, what goes into each pod is actually arranged by when it will next been needed by a picker, not by the product type.

In fact, each tower of shelving looks more like a jumbled mess than an orderly shelf at a supermarket. But the computer is keeping track, which is what matters.

Humans, armed with ever-present bar-code readers, log the goods at every step along the way so the computer can keep track of them.

Once filled, the squat, square 320-pound robots that sit under each pod lifted them up so they were just off the floor and whisked them away to the picking stations.

There, other Amazon workers consulted computer screens, which told them which items to pluck from the pod. The items were placed in yellow bins, one for each customer’s order.

This year’s hot products include less expensive tech tools such as Amazon tablets, anything voice controlled and all things Star Wars.

While Amazon at times has been criticized for the frenetic pace required of workers in its fulfillment centers, most seemed engaged and alert on this, the company’s busiest day of the year.

Clearly, effort went into making the day as fun as it could be. In the cavernous employee lunch room, a supervisor with a microphone was MC:ing a holiday trivia game, handing out prizes to those who could answer questions such as “What country is best known for its gingerbread?”

Once each order was complete, a complex superhighway of conveyor belts whisked the yellow bins to packing stations. There, the contents were placed in cardboard cartons, protected by plastic bags of air to keep them from jostling about, taped shut and labeled.

Those being gift-wrapped took a side trip through the wrapping station, where a line of mostly female employees carefully sized up each item. With remarkable care given their speed, they swathed each with bright wrapping paper, making military-precise creases as they went.

Once ready for shipping, the packages were hustled along another conveyor belt, where lasers read their tags, nudging them off the high-speed track down sloping and sometimes twisting chutes to a team busy packing another set of freight trucks, this time outgoing.

“The packers fit them in like like a huge, 3-D game of Tetris,” Gelman said.

The Tracy fulfillment center was one of more than 50 such Amazon centers across the United States busy picking, packing and shipping items on Cyber Monday.

“Some of these packages will get to customers today, if they were using Amazon Prime same day delivery,” said Gelman.

Regular customers have until Dec. 16 to order in time to get free shipping for Christmas, she said.

For Prime members, free two-day shipping ends on Dec. 22nd.

“So if you want to procrastinate you can, though I wouldn’t recommend it,” she said.

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