Sensors at the midway point of the table detect the angle, speed, and trajectory of the not-exactly-oncoming puck, and instructs the mechanism how and where to fire the disc -- you slap it back, and the same sequence happens on your compadre's end.

ENGADGET: Airhockey Over a Distance

At that point, it's possible to detect the faint beginnings of a smile, coming right through the piano.

NPR: Weary but Playful, The Little Willies 'Roll On'

Mulliner keeps close records of each vehicle's assembly, helping the production workers detect whether any flaws were made and at which point.

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