Dogs are mammals. They are warm-blooded, which means they generate the heat of their own bodies. They have fur, and mother dogs feed their young with milk produced by their bodies.
Dogs usually give birth to puppies about 63 days after the eggs are fertilized, and the size of the litter can vary. Although the average litter includes about six puppies, small dogs have smaller litters, while larger dogs tend to have larger litters. Very young mothers and older mothers also tend to have smaller litters. Puppies from smaller breeds tend to reach adulthood earlier than larger breeds, such as Great Danes.
Puppies begin to nurse immediately after they're born and begin to be weaned when they're about a month old. At this point, the mother might start to regurgitate solid food for them. If a puppy is healthy, it grows very quickly after birth and is actively exploring by the time it's 3 months old. This is also a good time for a puppy to become comfortable with humans.
Despite being a mammal, a newborn puppy can't regulate its own body temperature and therefore needs to huddle with its siblings and mother to keep warm. A puppy's fur can change color as it matures.