最后，约6000年以前，即公元前4000年，他们获得了一种新的更方便的制造工具的方法：他们发现了金属。
Not all of them at once, of course.
当然不是一下子同时发现了所有的金属。
It began with some green stones which turn intocopper when melted in a fire.
首先是那些绿色的石头，人们用火熔化它们，它们就变成了铜。
Copper has a nice shine, and you can use it to make arrowheads and axes, but it is soft andgets blunt more quickly than stone.
铜闪着美丽的光，你可以用铜锻造箭头和斧子，但是铜很软，它比石头更容易变钝。
But once again, people found an answer.
人们再一次发现了解决方法。
They discovered that if you add just a little of another, very rare, metal, it makes the copperstronger.
他们发现：如果你把另一种很罕见的金属掺和进去，便能使铜变硬。
That metal is tin, and a mixture of tin and copper is called bronze.
这种金属就是锡，而铜和锡的混合物就叫做青铜。
The age in which people made themselves helmets and swords, axes and cauldrons, andbracelets and necklaces out of bronze is, naturally, known as the Bronze Age.
人类用青铜制造他们的头盔、剑、斧头和锅，还有青桐制作的手镯和项链，这个时期，自然就被称为青铜时期。
