Entropy: Universe Tends Toward Disorder
Murphy's Law states, "Anything that can go wrong, will go wrong." Entropy is sort of like Murphy's Law applied to the entire universe.
Put simply, entropy is a measure of disorder, and the Second Law of Thermodynamics states that all closed systems tend to maximize entropy. Reversing this ever increasing tendency toward disorder requires the input of energy. That's why housekeeping is so tiresome. Left on its own, your house would get dusty, spiders would move in, and eventually, it would fall apart. However, the energy put into preventing disorder in one place simultaneously increases it somewhere else. Overall, the entropy of the universe always increases.
Entropy also manifests in another way: There is no perfect transfer of energy. Your body (or a cell) cannot perfectly utilize food as an energy source because some of that energy is lost forever to the universe. So, just like in finance, every transaction comes with a tax. (University of Washington microbiologist Franklin Harold liked to call it "God's energy tax.")
The common adage that "nothing in life is certain except death and taxes" hence takes on a new meaning.
Source: Galileo's Finger: The Ten Great Ideas of Science by Peter Atkins
Image: NASA/Wikimedia Commons