Before you look, a quantum particle can be in several states at once — the puzzle is that the rules seem to say a big object could be too.

Quantum rules, applied at a large scale, generate absurd paradoxes like a cat both alive and dead.

Schrödinger imagined a cat sealed in a box, linked to a random quantum event. By the strict rules, the cat would be, on paper, both alive and dead until you open the box. He did not actually believe this — he wanted to show how strange the theory gets at a large scale.

Why it matters The paradox is still used today as a test for the limits of quantum computers.