In 1935, Erwin Schrodinger, proposed an experiment where a cat is placed in a box with a sealed vial of poison that will break open at a random time. Now, since no-one knows when or if the poison has been released, until the box is opened, the cat can be thought of as both alive and dead. That's the basic principle - it's actually about quantum physics and is way beyond my understanding. Hope that gives you a handle on it :)
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