One of the best books on puppies is Another Piece of Puzzle: Puppy Development by Pat Hastings and Erin Ann Rouse. The book includes Kathryn Lanam's article on behavioral development of puppies. According to the article the 5th week (28-35 days) is a second awareness/identification week.
During this week puppies need physical and mental challenges, and their play behavior becomes much more sophisticated. I could see today that pups are making a really fast progress. They started to use the box with wood chips to go to bathroom. It is not easy for them as they have to climb out of the box to do it, then they have to climb back. Of course exercise like this helps develop their strength and coordination.
We are having a really warm spell - yesterday the temperature was reaching 90, and today was around 75. These pictures were taken today when pups were playing outside on the grass. They were chewing on each other, on Keena and they tried to chew on me too. They are now also showing stalking behavior. They like to be touched, they wag their tails, and they snuggle. Some of them growl more than the others.