The ancient Greek philosopher Democritus (460-370 BC) made a careful study of the composition of matter. He came to a conclusion by analyzing and speculating a large number of natural phenomena. When he saw that plants grew particularly vigorously on manure, he wanted to know: did some small particles in manure enter the plants? When he found that salt was dissolved in water, it disappeared, but the water was salty. He wondered: Are the tiny particles of salt dispersed in water? When he saw the fish swimming in the water, he concluded that the substance of water must be composed of water particles. Only in this way, when the fish swim over, the water particles will spread to both sides and the fish will swim freely. From these natural phenomena, he came to the conclusion that all matter is composed of some hard and inseparable particles. He called this small particle an atom (Greek for inseparable). This is the earliest atomic concept in history.