One stops -- not on a dime, it turns out, but rather over a QR code stuck to the floor -- and allows the other to proceed, carrying inventory to a human worker who will pluck out an item, scan it and send it off for packing and shipping.

In this building the size of 28 football fields, containing four miles of conveyor belts and 15 million items awaiting customer orders from Northern California and beyond, the two limbless goods-moving machines are part of Amazon's 30,000-strong robot army.

Gliding in straight lines on a grid, separated from workers by chain-link fencing with signs warning people to keep out, the machines can lift and carry up to 750 pounds of retail products.

It has leased a fleet of 20 jumbo jets to further speed deliveries as an estimated 54 million Americans have flocked to its two-day-delivery Prime service.The company says its superhuman robots have created far more jobs than they've taken, but experts say that employment trend will reverse as machines grow increasingly sophisticated and climb ever higher on the job-skills ladder, bumping Homo sapiens to the side.

The company in May won a patent for robot-aided stowing that says, "some or all of the activities described as being performed by a human operator may be performed by automated mechanisms."

Amazon ORder-fulfillment CENTERS

U.S. fulfillment centers: 50

Roboticized fulfillment centers in U.S.: 15

Roboticized fulfillment centers in California: 3

Human workers: 90,000

Robots: 30,000

Fulfillment centers, global: 123

Amazon associate Michael Keele, of Tracy, scans items before stowing them in a portable storage unit to be carried away by an Amazon Robotics robot at the Amazon fulfillment center in Tracy, Calif., on Tuesday, April 12, 2016.

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