Puppies are eating machines. Since they eat so much, this makes them potty machines. They grow and develop rapidly at the puppy stage, and they eat lots of food, burn up tons of energy, and they need to eliminate often and usually go wherever they want. Puppies are like babies, they do not have developed bowel and bladder control and can't "hold it" as long as an adult dog.
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Puppies come with challenges. They need to be taught how to interact with people and other animals, go potty, stop chewing and biting, and so many other behaviors.
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Puppies aren't totally untrained when you first pick them up. Their moms keep the area clean and free from puppy contamination until her puppies are old enough to follow her out of the den to relieve themselves. A mother's training is the beginning of a puppy's early training. Your puppy instinctively knows not to use the potty where they sleep.