Atomism teaches that all visible objects, which are composites or collections of linked atoms, have within them a good amount of void, though some have more than others. Perhaps you find it difficult to accept that hard objects are actually partially composed of void. Atoms that cause our sensation of heat are able to get through metals, atoms that cause our sensation of color are able to pass through glass, and so forth. The answer, according to the atomists, must be that these objects have different quantities of void within them. If atoms were of just one kind, that is, if they all had the same shape and size, then we would not experience the variety of nature which we actually do experience.
Different effects require different causes.
For example, sweet tastes are caused by round atoms, of a good size. Sour tastes are caused by atoms that are bulky, jagged, and many-angled, without curves. Pungent tastes are caused by atoms that are round, fine, angular and crooked. Salty tastes are caused by angular atoms which are of a good size, twisted, with two sides equal. Bitter tastes arise from atoms that are curved and smooth but very crooked and small in size.