A comet is a small world which scientists sometimes call a planetesimal. They are made out of dust and ice, kind of like a dirty snow ball.Comets come from two places: The Kuiper Belt and the Oort Cloud.
A comet will spend billions of years in the Kuiper Belt or Oort Cloud. Sometimes two comets will come very close to each other, or even crash into one another. When this happens the comets change directions. Sometimes their new path will bring them into the Inner Solar System.
This is when a comet begins to shine. Up until now the comet has been among millions of others exactly the same, but as they approach the warmer Inner Solar System they begin to melt leaving behind magnificent tails.
Unfortunately, comets don't live very long once they enter the warmer part of the Solar System. Just like a snowman melts in the summer, comets melt in the Inner Solar System. Although it is the most glorious part of their lives, traveling through the Inner Solar System eventually kills them. After several thousand years they melt down to a little bit of ice and dust, not nearly enough to leave a tail. Some even melt away completely.
