The animation is done via a HTML5 canvas element. They load in the D3.js library on the page, and the source for the actual animation also: https://students.br

The animation is done via a HTML5 canvas element. They load in the D3.js library on the page, and the source for the actual animation also: https://students.brown.edu/seeing-theory/js/balls.js https://students.brown.edu/seeing-theory/js/balls.js From the attribution in the comment, the effect comes from this: https://bl.ocks.org/mbostock/3231307 https://bl.ocks.org/mbostock/3231307