translation.en: 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.
translation.zh: 但想象一下,如果这全是实心的骨头, 我们的头会变得非常重, 我们将不能抬起头, 我们也不能看到我们四周的世界。