Don't argue. It's the law.

Professor Kompressor's house was a complete mess. It didn't matter that Maud, the once-or-twice-a-week housekeeper, did her best to tidy up. The Professor was simply too good at picking things up, moving them about and leaving them where they weren't supposed to be. When Maud complained, the Professor muttered something about the "laws of thermodynamics" and that "disorder must increase". This sounded like complete gobbledygook to her, so Maud ignored the Professor and quietly carried on cleaning. She knew perfectly well that it was possible to keep a home nice and tidy. There was absolutely no reason why chaos should be allowed to reign.

                                                from "Professor Kompressor: The Mechanical Maid"

I think you're familiar with the idea. You spend hours tidying up; bedroom, living room or desk. After some effort it looks nice and tidy, and it seems incredible that it could ever have got into such a deplorable state. However, as soon as you turn your back on it the decay begins and before too long the mess has returned. Why does this happen?

            A common explanation from people with some understanding of physics, like the Professor, is that the "second law of thermodynamics" is at play. This sounds terribly serious, getting the law involved in everyday happenings. This kind of law is different, though. It is not the kind enforced by people in uniform. Rather, it is part of the rulebook for the Universe that is being pieced together by people in lab coats (although they may not actually be wearing them these days).

            Let's take a closer look at this particular rule. First we need to understand the main word a bit better. "Thermodynamics" is the name for the part of physics that describes processes involving hot and cold. It's basically a simplification, where very complex systems are described in terms of a small set of numbers describing the average behaviour.

            To understand how this works, imagine zooming in on a digital photograph. At first, on the large scale, the image seems perfectly smooth. As you take a closer view you can start to see the individual building blocks (the pixels). Finally, on the small scale, these building blocks dominate and you can't see the big picture any more. When you look at the original picture, your eyes average over the pixels and make the image appear nice and smooth.

            Everyday physical systems work in the same way. They are built from individual particles (atoms, or at the even finer level, quarks), but it is generally too difficult to keep track of the movement of each of these little guys. It is more practical to zoom out (average over a large number of particles) and focus on a few numbers that describe the collective behaviour. Information is obviously lost in this process, but one can often get away with ignoring this.

            The temperature of a system depends on how active the individual particles are. Basically, if they don't move at all then the temperature is zero. In a typical situation they whizz around madly. The temperature encodes the averaged energy associated with their motion.

            The laws of thermodynamics are the rules that describe how you measure the temperature and how it evolves as time passes. The famed "second law" deals with a somewhat mysterious quantity called the entropy. It used to be that scientists thought that heat was carried by a quantity known as the caloric. Hotter objects simply had more caloric in them. However, this idea didn't quite work out. Instead, the entropy took centre stage. 

            The entropy describes the amount of order (or lack of...) in a system. Think of the amount of papers, pens and various bits on an office desk, or perhaps toys on the floor in a playroom. If they're all put away neatly, then the system is ordered and the entropy is low. If they're spread in a random fashion, then the entropy is high. The second law of thermodynamics states that the entropy can never decrease (as long as you ignore outside influences). All systems become more random with time.

            The entropy law was a great breakthrough for science because it allowed people to work out why it is that heat flows from hot to cold and not the other way around.  An overall colder system is more random, and therefore more likely, than one where the particles bunch up in local hotter region. The second law led to the development of kitchen refrigerators, which are obviously great inventions, and is important for many other physics problems, like the evolution of the entire Universe, as well.

            What do we learn from this? All things tend to disorder and chaos, pretty much as the Professor suggested. The law says that entropy must increase, so any effort to tidy up is futile. Not quite! You can "break the law" by tinkering with the system. This is exactly what you do when you spend your precious energy tidying up. You can lower the entropy of a room, desk or whatever by using your free will to expend some energy. Thermodynamics says that there is no such thing as a free lunch. Ultimately you can't win, but you can chose to spend some energy getting things back in order for a while.

            It's such a shame! I can't use thermodynamics to excuse the mess on the desk in front of me, so I guess I have to admit that I am too lazy to do anything about it.