Metal
No-one knows when the first metal was found and used. It was probably what is called native copper, which is sometimes found in large lumps on the ground. People learned to make this into copper tools and other things, although, for a metal, it is quite soft. They learned smelting to get copper from common ores. When copper was melted over fire, people learned how to make an alloy called bronze, which is much harder and stronger than copper. People made knives and weapons from bronze. This time in human history, after about 3300 BC is often called the Bronze Age, that is, the time of bronze tools and weapons.