Why is this the answer?

The answer is earth because this amazing riddle is like a secret code describing our own home in space. It gives us five big clues, and they all point right to the planet Earth. Let us look at the first line, "I am the third from a sparkle bright." The biggest, brightest sparkle in our sky is the Sun, which is a star. If you count the planets starting from the Sun, you will find that Mercury is first, Venus is second, and our planet, Earth, is the third. That is a perfect match! The next clue says, "I thrive throughout the day and night." This is talking about life. Earth is the only place we know of that has plants, animals, and people growing and living every single moment, all around the clock. Then, the riddle mentions something a little tricky: "Deep in the path of a cows white drink." This clue is a clever, old-fashioned way to talk about the huge galaxy that our solar system is a part of. We call it the Milky Way, which sounds like a path of white milk. Earth is right inside that giant, swirling group of stars. The fourth clue says, "I've had thousands of millions of years to think." Scientists know that the Earth is very, very old. It took billions of years for our planet to form and for life to finally start here. Finally, the riddle ends on a serious note: "But one of my creatures is killing me." This is a sad truth about people, the creatures who live on Earth. When we pollute the air, the water, and the land, we hurt the beautiful planet that gives us everything we need. The riddle is asking us to think about how we can take better care of our world.