dogs are quite intelligent , and have considerable vocabularies-but there 's a difference between knowing what 'sit ' and 'fetch ' mean and being able to say 'sit ' and 'fetch ' yourself , and no dog since time began has ever been able to manage anything better than 'woof and surely you know that chimpanzees and gorillas can be taught to communicate quite well , through signs and gestures-but they ca n't shape words any more than dogs can .