Why is this the answer?

The answer is heart because this is a super clever riddle that uses a trick to make you think about two different ideas that are both perfectly described by the same word. It makes you think about your feelings and also about the real muscle inside your chest that keeps you alive. Let's look closely at the first clue: "I'm the source of all emotion." When you watch movies or read stories, you often hear people talk about how their feelings come from their heart. For example, if a person is very kind and caring, they are often described as having a big, warm heart. If a friend is filled with love and happiness, we say they feel it deep in their heart. While scientists know that your amazing brain is actually what creates all those emotions like excitement and sadness, the word "heart" is the special, traditional way we talk about the place where love, bravery, and friendship live. That’s why this clue is a fun, gentle trick; it points to the emotional idea of the heart. Now we can figure out the second part: "but I'm caged in a white prison." This clue is talking about the real, pumping heart that beats inside you every second. Where is your heart located? It is safe and sound inside your chest. What protects it? A set of strong, curved bones called your rib cage. These bones wrap all the way around your heart and lungs, making a protective shield. Since the bones of your rib cage are white and they hold your heart safely inside like a protective enclosure, the riddle calls them a "white prison." This sturdy cage is very important because your heart is one of the most vital muscles you have, and it needs that bony armor to keep it safe from any bumps or accidental hits while you are running and playing. Putting the two clues together, both the figurative heart that stands for all your wonderful feelings and the actual heart, which is a muscle protected by the white rib cage in your body, are perfectly described by the single word, heart.