Oh, an invisible fence is like an electric fence. The dog in the yard wears a radio collar. Lines of electrically charged wires are set under the ground at the perimeter of the property. The dog is trained that if it goes up to a certain area close to the wires, it will get an electric shock through the collar. It works only if the dog is trained to respond to the shock, else the dog will easily just get shocked, and run through. It's like an invisible line in the sand. I don't think this dog was properly trained to avoid the area beyond the invisible fence. My second dog still at vet 3 hours after bringing it in. It's 8:45. Ugh.
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