Why is this the answer?

The answer is skeleton because this riddle is a very clever way to describe the amazing set of bones that live inside all of us. Let us look at each clue one by one to find the secrets hidden in the words. The first clue says, "I am whole but incomplete." Think about what a skeleton is. It is the entire, whole framework made up of all your bones. It has every bone from your toes to your skull, so it is a whole thing. But it is incomplete because it is missing all the soft parts that make up a living person, like the skin, the muscles, and the organs. It is only the hard, bony parts of what was once a living creature. The next line is, "I have no eyes, yet I see." If you look at a skull, you will notice two big, dark holes where the eyes used to sit. That means a skeleton has no working eyes. However, the skeleton helps doctors, teachers, and students see and understand how the body is built and how it moves. By studying the bones, we are able to "see" the structure of the body. The riddle then says, "You can see, and see right through me." A skeleton is not solid like a rock. There are many spaces between the bones, especially around the rib cage and the spine. If you stand behind a model of a skeleton, you can easily look right through those spaces. This clue also hints at X-ray machines. Doctors use X-rays to see through your skin and muscles to look directly at your skeleton. Finally, the riddle ends with a clever clue about size: "My largest part is one fourth of what I once was." Your skeleton, all your bones put together, is actually the largest single system of parts in your body. It makes up about one fourth, or 25 percent, of your entire body's weight. So if you were once a whole body, the skeleton is the biggest part left behind, and it weighs about one fourth of that original weight. All these smart clues fit perfectly together to point to the skeleton.