This is what's written in Arthur Spiderwick's Field Guide to the Fantastical World Around You about Giants. These lumbering brutes hibernate for most of their adult lives, sometimes sleeping for so long their backs become densely forested. This, coupled with the fact they can grow as large as hills and are often indistinguishable from the landscape, means it's entirely possible for a person to walk across the back of a sleeping giant and not even know it.' Highly territorial, giants seldom form