They can tell you if something is moving but they don't necessarily know if it's a human being or if it's your kids — they don't know the difference, so they will warn you all the time.
While Kearns warns that the setting was artificial, he says it's possible that greater persuasive power could lie with well-connected individuals in the everyday online world too.
Why will you die, thou and thy people by the sword, and by famine, and by the pestilence, as the Lord hath spoken against the nation that will not serve the king of Babylon?
The second sell-off happened last week, when Mr Semel warned investors at a conference hosted by Goldman Sachs that growth in online advertising was not quite what he had hoped.