Schrödinger's cat

It was a thought experiment which describes a paradox of quantum mechanics with an cat as representor of the macroscopic world. This experiment was described by Erwin Schrödinger in 1935.

The main question in this experiment was, is the cat alive or dead? The scenario presents a situation where the cat could probably be both simultaneously, dead or alive. This state is known as quantum superposition.

The experiment contains of an box, a cat, a flask of poison, a radioactive source in an sealed container, and an Geiger counter. If the Geiger counter detects radioactivity, the flask will be destroyed and the cat will be killed. Nobody knows when the radioactive source will collapse and as long as nobody looks into the box, the cat is both dead or alive. 

Schrödinger’s famous experiment poses the question, when a quantum system stops existing as a superposition of state and become one or the other.