system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: how do chaos pendulums work?

I've seen some cool videos about chaos pendulums on YouTube and whatnot but I can't think about why they're so erratic and never has the same pattern twice. Why is it so spontaneous and to what extent is it random?
gpt
It's not that chaos pendulums are random -- they certainly aren't.  We can write down equations describing chaos pendulums.  First, the solutions aren't sine-like in behavior.  But the real "problem" is that a small change in initial conditions or a small miscalculation of starting parameters (mass, length, moment of inertia, etc...) lead to very, very different long-term behavior.   It's actually possible to calculate what the next couple swings of the pendulum will look like, but beyond that the uncertainties lead to unpredictable behavior.  It's sort of like hurricane prediction.  For the next day we have a pretty good idea of what a hurricane's path will be.  But a week out, it's very difficult to predict because small differences in conditions will lead to very different behavior.

In general, the idea that small changes in initial conditions (or the inability to exactly describe a system) leads to unpredictable long-term behavior is the definition of a chaotic system.