are said to have a self-organizing phase. It is useful to distinguish three forms of emergent structures. A first-order emergent structure occurs as a result of shape interactions (for example, hydrogen bonds in water molecules lead to surface tension). A second-order emergent structure involves shape interactions played out sequentially over time (for example, changing atmospheric conditions as a snowflake falls to the ground build upon and alter its form). Finally, a third-order emergent structure is a consequence of shape, time, and heritable instructions. For example, an organism's genetic code affects the form of the organism's systems in space and time. Nonliving, physical systems In physics, emergence is used to describe a property, law, or phenomenon which occurs at macroscopic scales (in space or time) but not at microscopic scales, despite the fact that a macroscopic system can be viewed as a very large ensemble of microscopic systems. An emergent property need not be more complicated than the underlying non-emergent properties which generate it. For instance, the laws of thermodynamics are remarkably simple, even if the laws which govern the interactions between component particles are complex. The term emergence in physics is thus used not to signify complexity, but rather to distinguish which laws and concepts apply to macroscopic scales, and which ones apply to microscopic scales. However, another, perhaps more broadly applicable way to conceive of the emergent divide does involve a dose of complexity insofar as the computational feasibility of going from the microscopic to the macroscopic property tells the 'strength' of the emergence. This is better understood given the following definition of (weak) emergence that comes from physics: An emergent behavior of a physical system is a qualitative property that can only occur in the limit that the number of microscopic constituents tends to infinity. Since there are no actually infinite systems in the real world, there is no obvious naturally occurring notion of a hard separation between the properties of the constituents of a system and those of the emergent whole. As discussed below, classical mechanics is thought to be emergent from quantum mechanics, though in principle, quantum dynamics fully describes everything happening at a classical level. However, it would take a computer larger than the size of the universe with more computing time than life time of the universe to describe the motion of a falling apple in terms of the locations of its electrons ;