Your pup is very inquisitiveand interested in exploring the world around him.  Right now his brain is geared toward soaking in everything it can.  This coincides with a crucial phase in his development:  He is in what is called the critical socialization period. Through 4 months of age is the single most important period influencing how your dog will behave as an adult.  Now is when the foundation is laid for the rest of his life in terms of his attitude toward you, other humans, and other ...

Your puppy's senses and motor skills are well developed now.  He still needs to sleep a lot, but he's full of energy when he's awake.  His bladder and bowel control are continuing to improve.  His adult teeth are still coming in.  While this is happening, sometimes puppy teeth don't fall out as they should.  Have your vet check your pup's mouth on the next visit. 

Special thanks to AKC for this great information.