How can he be so bendy?

This is Sam now 18 weeks old; he sleeps like this a lot. When he is not asleep he is tearing around the place leaving a trail of devastation behind him. We had both forgotten how much time puppy rearing takes and the impact that has on an otherwise ‘normal’ life. Our terrier Millie, a spritely 10 year old Fell, either ignores him completely, growls at him in a ‘rip your throat out’ sort of way, or boxes with him around the house, sometimes for twenty minutes or more. Not sure what her view of him is.