**Chaos Theory** is a very important area of mathematics which can **explain a lot of what we see in the real world**. A **pendulum with one mass** is relatively **easy **to explain mathematically, and it **behaves nicely**. However if you put **another mass** in there, it **behaves chaotically**. Technically, this means that if you change the starting positions only **slightly**, the state of the system a short time later can change **drastically**. The **weather is chaotic**- a small error in measuring it today could be the difference between rain and no rain in a weeks days time. Watch these **two pendulum systems quickly diverge**, though they both start off with nearly the same settings. [more] [code]