So the first post from me is going to be about Physics and about quantum mechanics. The best known thought experiment in quantum mechanics is probably Schrödinger's cat. But before I go into details, I need to explain some general physical concepts.

What means Quantum Machenics?
Normally every object has a position and a size and maybe a speed. I need to know all those quantities, to determine what is going to happen to the object. For example when I throw a ball, I want to know where it touches the ground. If you tell me its position, speed, mass and maybe some other quantities, I can do that. In quantum mechanics that is different: you cannot know all those properties and therefore you cannot determine the exact process. That is why scientists talk about probability, that leave room for uncertainty. A particle has a chance of being in a position and having a specific speed. Maybe the speed is little bit faster or slower, but it is somewhere close. Now my prediction is also only going to be a probability. In other words, instead of saying a quantum is at Position A with speed v, you say it is near position A and roughly at speed v. It works this way for all properties a quantum has. An entity, which consists of such quantum particles, looses those properties when it reaches a certain size. Therefore quantum effects don´t work on objects like cars anymore. You have to consider far smaller particles in order to observe quantum effects. But for better understanding let´s consider a cat were effected by quantum effects.

What about Schrödinger's cat?
Now let´s come to Schrödinger's cat. This scenario presents a cat, that has only the property of being alive or dead. So there are only two states the cat can be in (like any normal being, it cannot be undead or sorts ...). In quantum mechanics you call that a superposition. It means there is a probability for each of those states. And because the cat is either dead or alive (and there is no third option), the probability of those two states added must be a 100%. For example as long as the cat is in the house, the probability of being alive (P_alive) is 100% and the probability of being dead (P_dead) is therefore 0%. Those probabilities might change, when the cat leaves the house. The probabilities might change to P_alive = 70% and P_dead = 30% or something like this. Important is that as soon as the cat is back home again, you know whether it lives or is dead. But during the rest of the time, you don´t know for sure. Some People say during this times when the cat could be alive or dead, it is both: alive AND dead. This is because it possesses a chance of being dead and alive. So the cat has both quantities (P_dead and P_alive) so it is dead and alive.

The thought experiment:
If you put a cat inside a box with a poison, which is released at a random time, you cannot know if the cat is still alive after some time. The poison might have been released or not. Therefore the cat might be dead or still alive or in other word the cat is dead and alive at the same time. The probability P_alive might be 50% and P_dead also 50%. That is called a paradox, because in common sense it is not possible to be alive and dead at the same time. The situation is resolved as soon as you look into the box and see the cat either alive or dead. At this moment one of the probability goes to 0% and the cat looses that property. The cat becomes alive OR dead again.

Why is that important?
Once you understand this example, you can apply this to other properties. For example think about a computer. A normal computer works with bits which can have one of two states: 1 or 0. Quantum mechanics would tell you that one bit has a probability of being 1 and a probability of being 0, as long as you don´t measure it exact state. Therefore the bit could also be in a state where both probabilities are 50%. That means in practice a quantum bit can switch not only between 2 states but 3 or more. If anyone is interested in that topic I can do more about Quantum Mechanics, just vote it up or comment below.

Sources and further reading:
https://en.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat