Schrödinger's cat. Schrödinger's cat: a cat, a flask of poison, and a radioactive source are placed in a sealed box.
If an internal monitor detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison that kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when exactly quantum superposition ends and reality collapses into one possibility or the other. Reality Crowd » Blog Archive » Quantum Mechanics is Strange. What The Bleep Do We Know (7 of 12)
The Arrow of Time. We are all aware of an intuitive "flow" of time from past to future.
Not only do we feel this flow of time, but we also see it manifested in the behaviour of objects which change over time. Many objects seem to behave differently in the forward time direction when compared to the backward time direction. For example, we don't see a spilt glass of water jumping up and going back into the glass, we don't see a broken egg reforming itself.