I wonder what makes dogs enjoy chewing a bone so much

Actually, dogs don't enjoy bones any more than they enjoy sticks, wwh, unless there's meat or juice on them. At least, that's my experience. Which leads me to the next question: "Why do dogs enjoy sticks?".

And why do we say someone studying for a test is "boning up" for it? No-one believes a dog chewing on a bone is trying to improve his table manners. (Or do they?)