Fossils are the ramains of animals or plants. 化石是动物或植物的遗骸。

　　They are usually inside of rocks. 它们通常在岩体内。

　　They are typically more than ten thousand years old. 它们通常有一万多年的历史。

　　Fossils are made by sudden changes on the Earth, like volcanoes and tsunamis. 化石由发生在地球上的突变形成的，比如火山和海啸。

　　Animals and plants are buried in soil by the sudden changes. 动物和植物因这样的突变而被埋在了土壤中。

　　Then, they become rocks. 然后，它们就变成了岩石。

　　The oldest fossil is about 4 billion years old. 最古老的化石约有40亿年的历史。