All matter is made up of countless tiny particles whizzing around. These particles are extremely dense; matter is mostly empty space. Matter seems continuous because the particles are so small, and they move incredibly fast. Even people of ancient times suspected that matter is made of invisible particles. They deduced this from observing things like water, rocks, and metals. These substances are much different from each other. But any given material—copper, for example—is the same wherever it is found. Even without doing any complicated experiments, early physicists felt that substances could only have these consistent behaviors if they were made of unique types, or arrangements, of particles. It took centuries before people knew just how this complicated business works. And even today, there are certain things that scientists don’t really know. For example, is there a smallest possible material particle? There were some scientists who refused to believe the atomic theory, even around the year of 1900. Today, practically everyone accepts the theory. It explains the behavior of matter better than any other scheme. Eventually, scientists identified 92 different kinds of fundamental substances in nature, and called them elements. Later, a few more elements were artificially made.