However, Lucretius, describing atomism in his "De rerum natura", gives very clear and compelling empirical arguments for the original atomist theory. He observes that any material is subject to irreversible decay. Through time, even hard rocks are slowly worn down by drops of water. Things have the tendency to get mixed up: Mix water with soil and mud will result, seldom disintegrating by itself. Wood decays. However, there are mechanisms in nature and technology to recreate "pure" materials like water, air, and metals. The seed of an oak will grow out into an oak tree, made of similar wood as historical oak trees, the wood of which has already decayed. The conclusion is that many properties of materials must derive from something inside, that will itself never decay, something that stores for eternity the same inherent, indivisible properties. The basic question is: Why has everything in the world not yet decayed, and how can exactly some of the same materials, plants, and animals be recreated again and again? One obvious solution to explain how indivisible properties can be conveyed in a way not easily visible to human senses, is to hypothesize the existence of "atoms". These classical "atoms" are nearer to humans' modern concept of "molecule" than to the atoms of modern science. The other central point of classical atomism is that there must be considerable open space between these "atoms": the void. Lucretius gives reasonable arguments that the void is absolutely necessary to explain how gases and liquids can flow and change shape, while metals can be molded without their basic material properties changing.