Why do armpit and x ray decide caveman is tough in the book holes?

Armpit and X-Ray do not decide that the caveman is tough in the book Holes. In the book, the boys discover a chest containing the remains of a Native American who was killed by the white settlers. They name him "Stanley Yelnats" after the protagonist of the story, and they eventually learn that he was a strong and brave warrior who fought for his people.