The planets aren't the only wanderers  of the sky. There're some things even farther and longer. Long after people were no longer afraid of planets, they still thought the comets  were bad objects that traveled in the sky. They were afraid that the comets would suddenly land on the earth and hurt them. But, in fact, even if a comet did hit the earth, nothing much happened. A comet is really a harmless  thing. Comets have round heads and long tails , and they are very bright. Sometimes they look like heads with long hair flying with them. That's why they're called comets. Comet means long-haired. Like the stars and planets, comets are far away. But we can know its size. The head of a small comet is as wide as the Pacific Ocean. And a big comet's head is ten times as big as the earth. Its tail is millions of kilometers long. Then why wouldn't the great big thing hurt us if it hit the earth? Because it isn't solid. It's thin like cloud. Its tail is nothing but bright gas. And its head is made of small pieces and they could not hurt the earth. Some comets can be seen for a few months. Then they go away. Some of them come back in a certain number of years. Some are never seen again. Perhaps these comets that never return have broken into pieces. For some _ think that shooting stars  are pieces of broken comets. From the passage we know the tails of comets are made of    _  .
water solid gas small stones