The Belousov–Zhabotinsky reaction (often shortened to "BZR" or "BZ") is an example of a chemical oscillator. It is often used to illustrate a chaotic system. The BZR is made of several chemical reactions, that occur in sequence. It shows chemical oscillation, which is unusual for chemical reactions. At first it was thought to be a measurement error, or an artifact. It was thought that according to the Second law of thermodynamics, this should not occur. According to this law, a system that is in an unordered state will not assume an ordered state on its own, that is without the provision of energy. However, this is only applicable in a closed system, and does not apply to the BZ reaction.