Said cat is placed in a sealed box. Attached to the box is an apparatus containing a radioactive nucleus and a canister of poison gas. When the nucleus decays, it emits a particle that triggers the apparatus, which opens the canister and kills the cat.
While the box is closed, no observation has taken place. According to the Copenhagen Interpretation, the system exists in a mixed superposition of the states "decayed nucleus/dead cat" and "undecayed nucleus/living cat." Only when the box is opened and an observation performed does the wave function collapse into one of the two states.
This is, of course, silly; there is no such thing as a superposition of living and dead cats.
The nucleus and the cat are an example of an entangled state. The many-worlds interpretation of quantum mechanics handles this thought experiment gracefully; since the cat has many more degrees of freedom than the nucleus, the living and dead states decohere?.
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