Zipf
language obeys a law nobody wrote.
stream any text — word #2 appears half as often as #1, always
As the text streams in, the count for the most common word roughly doubles the count for the second, triples the third, and so on — a 1/rank pattern nobody wrote into the language, it just falls out of how words get used. The same shape turns up outside language too, in things like city sizes and incomes.
Concepts: language, statistics.
An interactive you play with one thumb, on Wundr.