The big surprise about dogs is how many words they can learn—the average service dog knows at least 50 commands, and a dog called Chaser knew over 1,000 names of objects and toys. Chaser was truly extraordinary because she could learn words through a process of exclusion. When she heard a new word, she inferred that it was for a new toy. So she brought back a toy that she had not yet learned a name for. Chaser remembered all the words she was taught and her vocabulary just kept growing—she only stopped at 1,000 because her owner had no more time to train her. As remarkable as dogs are at understanding us, this is just one side of communication. Does the conversation go both ways? Dogs have fairly plastic vocal chords or a ‘modifiable vocal track.’ They can subtly alter their voice to produce a wide variety of different sounds with different meanings. In an experiment, researchers recorded ‘alone barks’ of dogs when they were alone, and ‘stranger barks’ when a stranger was approaching. Then they played this bark to people to see if they could distinguish the difference.