One pendulum swings like a clock. Hang a second one from it and the swing turns wild: it never repeats, and the smallest nudge at the start changes everything later.
Start with one arm: a plain pendulum, as regular as a clock. Now choose two arms and watch the tip: loops, flips and lunges that never quite repeat. Then pick Twins: two double pendulums whose starts differ by a thousandth of a degree, far less than you could ever measure. For a few seconds they move as one. Then they part company, and soon they have nothing to do with each other. The readout at the bottom counts how far apart they've drifted.
Pause and drag any weight to set a new start. Try Two arms, swinging gently: with small swings the double pendulum is nearly tame, and the twins stay together much longer. Chaos needs energy. Add friction and everything slowly runs down to hanging still.
This is chaos: a system that follows exact rules, with no randomness at all, yet can't be predicted far ahead, because any tiny difference in where it starts grows and grows. The weather is chaotic in the same way, which is why forecasts fade after a week or two. In 1961 the meteorologist Edward Lorenz restarted a weather simulation from numbers he'd rounded to three decimal places and got a completely different forecast; he later asked whether the flap of a butterfly's wings in Brazil could set off a tornado in Texas, and the "butterfly effect" got its name.
Under the hood, the page solves the real equations of motion for a chain of rods and weights, stepping a thousandth of a second at a time. The energy kept readout is how we know the maths is honest: with no friction, a real pendulum keeps all its energy, and so does this one, to four decimal places.
