The second law of thermodynamics

The second law of thermodynamics states that every spontaneous process causes an increase in the entropy of the universe.

A popular example is the fact that your room seems to get messed up all by itself (spontaneously), whereas tidying up requires energy. Why is that? Well, when you decide that the orderly place for your bag to be is on the hook on the door, then every other place increases the disorder of your room. Since there are many more disorderly places for your bag (and every other item in your room) than orderly, the chances of disorder are simply higher and the increase in entropy is a matter of statistics.

Title reads Entropy, Second Law of Thermodynamics. On the left is an image of a tidy living room. On the right is an image of the same living room but messy with papers and clothes everywhere and the furniture not aligned. Arrow from tidy to messy reads spontaneous reaction. Arrow from messy to tidy reads requires energy.

Figure 1: Increasing the entropy of your room is a spontaneous process, whereas tidying up requires energy.

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Entropy