Most dogs understand human speech,rhythm, patterns and inflections to a great deal, in addition to our body language, in conveying what our intent is to our dog. They are capable of learning commands if someone takes the time to teach them in a consistent manner for each one. Once a dog becomes proficient in understanding that word, many can discern the word if spoken in a sentence. This ability gives the impression that the dog understands everything said in context to the words he knows. They really do try to understand exactly what we want and many make rather educated guesses in their attempts. There are a few exceptional dogs who are capable of building very large vocabularies over their lifetime. These dogs are very perceptive and are considered geniuses in their own right. One dog, a Border Collie named Chaser, has a vocabulary of well over a thousand words. This is comparable to a three year old child’s vocabulary. While humans learn words by context, dogs must learn each one by itself from scratch. As a result, it takes dogs longer to learn words but they remember them. Dogs are able to also identify an unknown object from a group of known objects when asked to retrieve it. This ability to do so was thought to only be an ability of humans and is known as “fast mapping”.