The Cat that is both alive and dead.

In Schrödinger’s cat thought experiment (Schrödinger, 1935),  there is a cat locked inside a box along with a glass vial of cyanide; a hammer set to potentially break the vial; and a Geiger counter inside of which there is a sample of a radioactive substance small enough that there is a 50% chance of one of the atoms decaying in the course of one hour and a 50% chance that none will. 

If an atom decays the Geiger counter will click which causes the hammer to fall, the flask to break, the cyanide gas to be released and the cat to die. 

If an atom does not decay, the cat remains alive. 

If the inside of the box is not observed during this hour, Schrödinger took the formalism of quantum mechanics to imply that the cat would be in a superposition of being alive and dead