"The cat experiment as described by the Erwin Schrödinger 1935":
A cat is enclosed in a steel chamber, together with the following diabolical apparatus (which one must protect from the direct influence of the cat): in a Geiger counter there is a minuscule amount of radioactive material, so little that in the course of an hour perhaps one of the atoms decays, but it is equally likely none does; if it happens, the Geiger counter registers and effects the release of a small hammer, which shatters a bottle of poison. Once one has left this whole system to itself for one hour, one can then say that the cat is still alive if in the meantime no atom has decayed. The first decayed atom would have poisoned it. The psi-function of the whole system represents this in such a way that the living and the dead cat are (pardon the expression) mixed or smeared together.
(Schrödinger, Erwin. 1935. Die gegenwärtige Situation in der Quantenmechanik. Die Naturwissenschaften 23: 807-812; 823-828; 844-849.)