It’s got to be The Zipf's law.

It says that if you pick any language or a book and rank the most commonly used words then the frequency of any word is inversely proportional to its rank in the frequency table.

For example…

If the most used words in a book are ranked as:

1. The

2. Of

3. And

4. To

5. A

then the word "Of" will be used half as often as "The". Similarly the word "And" will be used (1/3) times as often as the word "The". All the words will follow the (1/rank) system.

The above image shows the graph of most used words in the book Moby Dick.

This law applies to every language, every book and many other things such as city populations, last names, number of phone calls people receive, etc.

I took two random answers from a site under the same question and made a word rank using the Internet.

Here the example of Zipf’s law can be seen; the word “you” is used almost 1/2 as often as the word “The” and the word “of” 1/3 times as often and so on up to rank 5 (because the data is taken from only 1000 words). The more words you use, the more accurate this law gets.

It’s amazing how a simple law can define such a complex thing.