하지만 또한 생각해보세요. 두개골이 빽빽하게 뼈로 채워져 있었다면, 머리는 엄청난 무게여서 바로 들지도 못할 것이고, 우리 주변을 바라보지도 못할 것입니다.
But also imagine if they were filled with solid bone -- our head would be dead weight, we wouldn't be able to hold it erect, we wouldn't be able to look at the world around us.