This is a simulation of a double pendulum system, plotted using the forward Euler method for approximating solutions to differential equations. Due to the chaotic nature of double pendulums and the fact that the forward Euler method is an approximate numerical method, this will be wildly inaccurate fairly quickly, but at any specific point in time it will look realistic.
This program was made on the same day as BallDemo, with the same friends. For reference, the top pendulum is a (its initial angle is α), the bottom is b (initial angle of β), and angles and angular velocity are in radians.
Interestingly, the nonsensical starting configuration α=1, β=1.5, vα=vβ=0, g=-10, mα=100, mβ=-1, lα=lβ=1 seems to be non-chaotic.