As early as 3 weeks old you can see the beginnings of individual personalities in the puppies.  Even though their eyes and ears open between 10 to 14 days of age, they do not really notice each other until 3 weeks old.  Their teeth are coming in by then as well.  Lots of development is happening all at once.  Their ears are getting bigger, coats are getting longer and they are toddling around their bed better each day.  They start playing with each other by trying to bite each other’s noses off.  So cute.  This is the time I open a door in their bed so they can come out and piddle in a litter box.