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I decided to give the boy the treat and I told him to have Trevor sit. He takes it, and kind of tentatively says, "sit". And I say, no - you have to make him think you're in charge.
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So he straightened himself up and said, "Sit" in a confident voice and Trevor sat and then took the treat the boy handed him.
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Now this works great because Trevor is dainty and doesn't just wolf down a treat, he takes it cautiously to make sure he likes it.
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The boy was excited and as he and his dad left the elevator I hear him say, "Did you see? That was the first time I ever did that and the dog sat!"