听力原文： I'm going to pass this piece of amber around so you can see this spider trapped inside it. It's a good example of amber-inclusion, one of the inclusions that scientists are interested in these days. This particular piece is estimated to be about 20 million years old. Please be extremely careful not to drop it. Amber shatters as easily as glass. One thing I really like about amber is its beautiful golden color. Now, how does the spider get in there? Amber is really fossilized tree resin. Lots of chunks of amber contain insects like this one or animal parts like feathers or even plants. Here is how it happens. The resin oozes out of the tree and the spider or leaf gets in cased in it. Over millions and millions of years, the resin hardens and fossilizes into the semiprecious(次珍贵的、准宝石的) stone you see here. Ambers can be found in many different places around the world. But the oldest deposits are right here in the United States, in Appalachia(阿巴拉契亚). It's found in several other countries, too, though right now scientists are most interested in ambers coming from the Dominican Republic（多美尼加共和国）. Because it has a great many inclusions, something like one insect inclusion for every one hundred pieces. One possible explanation for this it that the climate is tropical and a greater variety of number of insects thrive in tropics than in other places. What's really interesting is the scientists are now able to recover DNA from these fossils and study the genetic material for important clues to revolution. (39)