Schroedinger's cat (E. Schroedinger; 1935):
A thought experiment designed to revel the counterintuitive & strange notions of reality that come along quantum mechanics.
A cat is sealed within a closed box; the cat has ample food, air and water to survive an extended period. This box is designed like that no information (i.e., sound, sight etc.) can pass into or out of the box the cat is completely cut off from your observations. Also within the box with the poor kitty (in fact Schroedinger was not too fond of felines) is a phial of a gaseous poison, & an automatic hammer to break it, flooding the box & killing the cat. The hammer is hooked up into a Geiger counter; this counter is checking a radioactive sample and is designed to trigger the hammer killing the cat must a radioactive decay be detected. The sample is selected so that after, say, one hour, there stands a fifty-fifty chance of a decay occurring.
The question is that, what is the position of the cat after that one hour has elapsed? The spontaneous answer is that the cat is either dead or alive, but you don't know which till you look. But this is one of them. Quantum mechanics, conversely, says that the wavefunction defining the cat is in a superposition of states: the cat is, actually, fifty per cent alive & fifty per cent dead; it is both. Not till one looks and "collapses the wavefunction" is the Universe forced to select either a live cat or a dead cat and not something in between.
This show that observation also appear to be an significant part of the scientific procedure -- quite a departure through the absolutely objective, deterministic way things utilized to be with Newton.