Drop hundreds of balls through a triangle of pegs. Where do they end up piling?
A Galton board drops balls one at a time through staggered rows of pegs, each ball bouncing left or right at random every time it hits one. Dropped from a single point at the top, where would you expect hundreds of balls to pile up at the bottom: scattered evenly across the width, clustered at the two edges, or something else entirely?
Reveal the answer
The balls pile into a bell-shaped curve centred under the drop point, with fewer and fewer landing further out. Each ball's final slot is the sum of many independent random left-or-right bounces, and the central limit theorem guarantees that summing enough independent random steps produces a normal distribution almost no matter how each individual step behaves. Francis Galton built the device in the 1890s specifically to make that abstract statistical law something you could watch happen.