Why is this the answer?

The answer is mercury because this is a super clever riddle that uses one word to mean three completely different things. This type of puzzle is fun because it makes you think about science, history, and the amazing objects we use every day. To solve it, we simply need to look at each clue one by one. The first clue is, "I'm a god." Thousands of years ago, the ancient Romans believed in many gods and goddesses. One very important god was named Mercury. He was known as the official messenger of all the gods because he could fly incredibly fast. In old pictures, you can always spot him because he is wearing a helmet and sandals that have little wings on them to help him fly through the sky. So, the name Mercury is a link to a famous figure from history. The second clue tells us, "I'm a planet." If you look out into our solar system, the very first planet closest to the sun is also named Mercury. Since it is so close, it has to rush around very quickly in its orbit. It was named after the Roman god because they are both famous for being the swiftest things around. This tiny planet orbits the sun faster than any other planet. The last clue is, "I measure heat." This is where the word "mercury" means a unique material. There is a shiny, silver, liquid metal called mercury. This metal was once used inside the thin glass tubes of old-fashioned thermometers. When the temperature got warmer, the liquid metal would expand and rise up the tube, showing people exactly how hot it was outside. This clever riddle shows how one simple word can have a big meaning in three separate parts of our world: a fast-flying Roman god, the quickest planet in space, and a special material that helps us measure temperature. That is why mercury is the perfect, tricky answer to this wonderful riddle.