Schroedinger's cat (E. Schroedinger; 1935): A thought experiment designed to exemplify the counterintuitive and strange ideas of reality that come all along with the quantum mechanics.
A cat is sealed within a closed box; the cat has plenty air, food, and water to stay alive in an extended period. This box is designed in such a way that no information (that is, sight, sound, and so on) 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 (it seems that Schroedinger was not too fond of felines) is a phial of a gaseous poison, and an automatic mallet to break it, flooding the box and murder the cat. The mallet is hooked up to a Geiger counter; this counter is observing a radioactive sample and is designed to trigger the mallet killing the cat -- must a radioactive decay be noticed. The sample is selected so that after, say, 1 hr., there stands a 50-50 chance of a decay happening.
The question is what is the state of the cat after that 1 hr has gone? The intuitive reply is that the cat is either alive or dead; however you do not know which awaiting you look. However it is one of them. The quantum mechanics, on other hand, states that the wave-function explaining the cat is in a superposition of states: the cat is, however, 50% alive and 50% dead; it is both. Not until one looks and "collapses the wave-function" is the Universe forced to prefer either a live cat or a dead cat and not somewhat in between.
This point out that observation also appears to be a significant portion of the scientific procedure quite a departure from the extremely objective, deterministic way things employed to be with Newton.