Schrödinger's cat (thought experiment)

Schrödinger's cat is a thought experiment illustrating quantum mechanics' strange nature. It imagines a cat placed in a sealed box with a radioactive atom, a Geiger counter, and poison. If the atom decays, the poison is released, killing the cat; if not, the cat remains alive. Quantum mechanics says the atom is simultaneously decayed and undecayed until observed, so the cat is both alive and dead at the same time until someone opens the box to check. This highlights how quantum states are probabilistic and only become definite through observation.