Answer: Democritus was one of the "atomist" philosophers of his time.
Democritus thought that the atoms were indivisible, and that were distinguished by shape, place, size, and order. That "shape" that the atoms had, was the responsibility of the atoms "ensembling" with each other.
And the atoms will remain ensembled in a body until that body hits another one, and then the atoms will break free until the form another body.