Who's asking: Hayley Ehrenfeld, Cambridge, MA
Dogs are born in litters of anywhere from one to 15 puppies. The average size is between three and six puppies. These puppies are usually not identical to each other; each puppy starts as a separate egg and develops with its own placenta. In fact, puppies within a litter can even have different fathers (something I learned only recently), so they can look very different from each other.
For many years, people assumed that puppies were always fraternal twins. Fraternal twins (like my sister Kathy and me) are no more like each other than any other siblings.
In fact, puppies can be identical twins, but it's rare. Identical twins come from a single egg (monozygotic), and have the same DNA. Various environmental factors can make them look slightly different from each other, just as with humans. My younger sisters, for example, are identical twins, but even when they were younger, no one who was paying attention had trouble telling them apart.