"Moving in space, the atoms originally were individual units, but inevitable they began to collide with each other, and in cases where their shapes were such as to permit them to interlock, they began to form clusters. Water, air, fire, and earth, these are simply different clusters of the changeless atoms."

Source: Ancilla to the Pre-Socratic Philosophers: A Complete Translation of the Fragments in Diels Fragmente Der Vorsokratiker, translated by Kathleen Freeman, Harvard University Press, (p. 163), 1948.