Cat's Out Of The Bag....Box...Whatever
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.
If this sounds rather silly, you'd be in agreement with myself, Einstein, and the guy who came up with the thought experiment, Erwin Schrodinger, specifically to show the repercussions of applying the Copenhagen interpretation of quantum theory to the world at large. Common sense tells us that the cat is either alive or dead. In fact, taking this a little further, one can plainly deduce that sealing a cat in a box even without a killing device will eventually lead to a zero probability of the cat being alive.*
*Explosions Inc. does not condone sealing pets, children, or exes in boxes. ALWAYS make sure to use air holes.
To work around paradoxes like these, Hugh Everett developed the Many Worlds interpretation which gets rid of the problem of declaring exactly when the wave function collapses by doing away with the collapse altogether. Instead, both outcomes occur as the universe splits in two. True that solves the paradox but it seems an awful lot of work to avoid admitting you're just a horrible pet owner.
The problem with calling shenanigans on the idea of superposition however, is that it is completely possible to keep a single atom in that state. It requires extremely low temperatures and sometimes magnetic fields but the very idea of superposition is what is making quantum computers a reality.
There are ways to solve the above paradox that do not necessarily result in a myriad of alternate realities. The one that fascinates me the most comes from a book about information theory written by Charles Seife called "Decoding the Universe". One of it's main ideas is that information underlies pretty much everything and some type of physical transaction takes place every time information is transmitted.
When two atoms collide, information is traded. If you know the original velocity or position of one atom, after the collision you can glean an observation of the other atom by how the properties of the first atom change. This is why scientists have to isolate an atom to keep it in superposition. Otherwise it will collide with another atom and the uncertainty will be lost.
Now apply this to the cat in the box of uncertain death. Compared to an atom the cat is large and warm. Even if the cat was in a vacuum (and didn't need the air or pressure that an atmosphere provides) there would still be many atoms hitting that large, warm target. It may not be much but information is traded, an observation is made and the wave function collapses into dead or alive.
The point of all this is the issue of taking a scientific idea like superposition and applying it as a universal rule. To be sure, the math of quantum mechanics is solid, it accurately describes and predicts phenomena at very small sizes and it is why we have cellphones and other tiny electronics. But remember that the reason quantum mechanics was devised in the first place is because the existing "laws" of classical physics only apply down to a certain size. Past that point the model we've created doesn't accurately reflect the natural world. The same thing happens when things get appropriately big. Newton's ideas about gravity work perfectly well on the scales that we are used to but when we start examining truly massive structures Einstein had to devise a new model to explain why things work the way they do. The map is not the landscape, the model is not the thing. The cat is either alive or dead and, if alive, is probably going to make you pay for trapping it in that box. You sicko.