Border collies develop a biting habit since early age. Just as we like to inspect and touch things we're unfamiliar with - border collies go around exploring the world around them using the one thing they can control best - their mouth. They will "test out" different types of material by biting on it and see what happens. When playing with their litter mates, border collies learn to limit their biting power during play. If one of the litter mates bites too hard his brother will squeak and refuse to play. This way the puppy will learn that biting too hard on some things is not acceptable.