Why is this the answer?

The answer is coal because this clever riddle tells the complete story of how a piece of coal changes color as it is used to make a warm, cozy fire throughout the day. Many years ago, before people had electric or gas heaters, they relied on this hard, black rock every single day to heat their houses, cook their food, and stay warm. The riddle gives us three perfect clues that describe the coal's transformation over 24 hours. First, the riddle says, "black in the morning." This clue is about the new coal that has not been burned yet. When you look into a cold fireplace before the fire is started for the day, the coal sitting there is a deep, shiny, and dull black color. It is hard, just like a rock, and it is waiting for someone to light it up to begin its important work. The black coal is the fuel ready to go. Next, the riddle moves to the middle of the day: "red in the afternoon." This is when the coal is burning at its best. Once the fire is lit, the black coal slowly heats up, and the pieces at the center of the flames start to glow a bright, fiery red and orange color. The coal becomes glowing red embers that give off tons of heat and light. This hot, red stage is the peak of the coal's usefulness, warming the family and cooking their food during the busiest time of the day. Finally, the riddle describes the end of the process: "white at night." When the evening is over and everyone is tucked into bed, the red-hot embers cool down. As the fire completely goes out, the solid pieces of coal break down and turn into a light, powdery material. This material is called ash, and it is a pale gray or white color. The white ash is all that is left in the fireplace after the heat and color are gone, waiting to be swept out before the next morning's black coal can be put in to start the cycle again.