Why is this the answer?

The answer is charcoal because this clever riddle tells the entire life story of a piece of charcoal as it is used to make a warm, cooking fire. It is a wonderful puzzle that uses colors to describe three different moments in time. Let us look at the first clue: "black when you get it." When you first buy charcoal from the store, or when you find a piece of it, it is a hard, very dark black. Charcoal is actually made from wood that has been heated up in a special oven with very little air. This process takes out all the water and other things, leaving behind something called carbon, which is why it is so dark. So, when you are getting ready to start your fire, the new, fresh charcoal is completely black. Next, the riddle mentions: "red when you use it." This is the exciting part! When you pour lighter fluid on the charcoal and light it up, the pieces begin to burn. As they get super hot and ready for cooking, they stop being black and start to glow a bright, fiery, beautiful red. This glowing red color shows that the charcoal is working hard and making lots of heat. This red stage is when the charcoal is most useful, heating up the grill so you can cook delicious meals outdoors. The final clue is: "white when you're all through with it." What happens after the food is cooked, the grill is cooled down, and the charcoal has burned up all its energy? It changes one last time! The heat and the burning turn the black and red pieces into a fluffy, powdery substance called ash. This ash is usually a light gray or white color. It is a sign that the charcoal has done its job and its story is finished, leaving only the white leftovers behind. That is why charcoal is the perfect answer, changing from black to red to white in its amazing journey.