Why is this the answer?

The answer is mirror because this clever riddle uses fun word pictures to describe exactly how a mirror works. Let's explore each line of the riddle to uncover its secrets. The first line says, "When there is fire in me then I am still cold." Imagine you hold a mirror near a warm fireplace or a small candle flame. The mirror will show you a reflection of the bright, hot fire. It looks like the fire is right there, deep inside the mirror. But even though the mirror is showing the fire, the mirror itself is made of glass and metal, and it stays cool to the touch. It is only holding a picture of the heat, not the actual heat itself. The next line is: "When I own your true love's face then you will not see me." This is a tricky part about seeing reflections. When you or someone you love looks into a mirror, the reflection is so clear and perfect that your eye is completely focused on the image of the face. You are seeing the person, but you are not thinking about the smooth glass or the silver backing of the actual mirror. The mirror's job is to disappear behind the perfect image it shows, so you stop seeing the object and only see the picture. The third clue is very direct: "To all things I give no more than I am given." This means a mirror is a very honest friend. If you wave one hand, the mirror waves one hand back. If you look sad, the mirror looks sad back. It can never show you an extra arm, or a different color shirt, or a happy face if you are frowning. The mirror can only reflect, or give back, exactly what is given to it by the object standing in front. Finally, the riddle concludes with, "In time I may have all things, and yet I can keep nothing." Over the years, a mirror has "seen" a reflection of the sun, the moon, a flower, a book, and thousands of different people. It has shown a picture of almost "all things." But as soon as something moves away, the picture is instantly gone. The mirror's surface is wiped clean. It cannot save or hold onto any of the images it has shown. All these clever clues fit together perfectly to describe a common household object: the mirror.