Solve the mystery #10

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Detective Hobbs was on his way to the big Detroit auto show when he decided to visit his old friend, Dr. Sam. At Sam's home he was shocked to learn that just two days earlier his long time friend had hanged himself. "Sam was in excellent health and spirits when I heard from him last week," Hobbs told the sheriff. "I can't believe he committed suicide." "But he did---I investigated it myself," said the sheriff. "Here's how it all happened." "Pete Porter, Sam's manservant, was returning to the house late that night when he spotted a light in the attic. As Porter got out of his car, he saw through the open attic window Sam knotting a rope around his neck. The other end was tied to a rafter. Then he saw Sam calmly kick away the small stool he was standing on, and that was the end. "Porter found the house locked. He had forgotten his key; so he went to the neighbor to call me." "He repeated to me exactly what I've just told you," said the sheriff. "When I arrived at Sam's house, I had to force the front door open." Porter and I then ran up the three floors to the attic. Sam was dead. The coroner has no doubt that death was from hanging." "The attic floor was clear except for the little stool that lay overturned by the door," concluded the sheriff. "Let's go out to the house," said Hobbs. "From everything you've told me about Porter's story, I can tell you he's lying!" How did Hobbs know that Porter was lying?