What happens when the robots sound too much like humans?

Artificial intelligence has a new challenge: Whether and how to alert people who may not know they're talking to a robot.

On Tuesday, Google showed off a computer assistant that makes convincingly human-sounding phone calls , at least in its prerecorded demonstration. But the real people in those calls didn't seem to be aware they were talking to a machine. That could present thorny issues for the future use of AI.

Among them: Is it fair — or even legal — to trick people into talking to an AI system that effectively records all of its conversations? And while Google's demonstration highlighted...