I am writing this article because a lot of nice pups are given back or worse, put down because the handlers simply did not know how to properly control them. Hard pups are those that get corrected for being snotty and turn right around, after the correction, and go back to being snotty again. An example would be chewing on the kids or biting too hard on the older geriatric house dog. Do not confuse this description with normal puppy behavior. All puppies like to play.
All normal puppies are bouncy, mouthy, and rambunctious.
These hard puppies seem driven. They have a very high pain tolerances and recover very quickly from a correction. The vast majority of people have never seen a really hard puppy because they are rare. Hardness is not breed specific, nor is it size specific. I have seen some of the terrier breeds produce very hard pups.