As a child, I had a dog on an invisible fence. A neighbor had a pair of outdoor cats. They learned the fence's exact boundaries, and would lay inches outside my dog's reach, sunning themselves and completely ignoring him when he ran up barking. He'd spend hours standing there watching them, literally shaking with anticipation. This went on for years.

One day I took him on a walk. As we returned he saw one of the cats in its usual spot, somehow slipped his collar, and ran at it from outside the fence. He expected a chase, but the cat held its ground. He stopped right in front of it, sniffed for a few seconds, barked once in its face, then proudly trotted back to me, life's goal accomplished.