It starts completely random, but eventually favors one side
There is one red ball and one blue ball in an urn. Draw one ball, put it back, and add one more ball of the same color. What happens if you repeat this? Each draw is random, yet one color usually ends up becoming overwhelmingly more common.
Why does it become biased? Once one color becomes more common, its chance of being drawn next also increases, so the process becomes increasingly biased, like a "rich get richer" effect. But which color wins is completely random — run it many times and a different color can win each time.
Red 1 ballBlue 1 ballRed share 50%
💡 This model is one of the simplest mathematical models of the "rich get richer" effect (such as follower concentration on social media, population concentration in cities, or the popularity of a particular language). It shows how a small coincidence early on can create a huge difference later.