Let’s say you’ve sealed a cat in a box with a killing device powered by the radioactive decay of an element (Don’t ask why. In this hypothetical you’re a sociopath). If the element decays, the device is triggered, the cat dies, and the ASPCA will come gunning for you. Whereas the average decay rate of elements is well known, exactly when each atom actually decays is not able to be predicted. The Copenhagen interpretation of quantum theory states that until an observation is made of the inside of the box, the quantum system described above is represented by a wave function in superposition. That is, the cat can be considered both alive and dead.