 So the concept of quantum randomness is essentially linked to the concept of measurement in quantum mechanics. Before quantum mechanics was developed, it was generally acknowledged by scientists that if you have a system that is a well-determined state and you perform a measurement on it, you're going to get a very well-determined outcome. Now in quantum mechanics that is no longer the case, you can have a system that is in a perfectly well-defined state and yet when you perform certain measurements on it, you can get random outcomes. So in order for me to illustrate that concept, I need to bring the concept of superposition. Superposition is a concept that is not inherently quantum. It's based on a system displaying several linear combinations of states simultaneously. And you can think of it as, for example, a radio wave carrying many different frequencies. There is nothing inherently quantum about that. What gives the quantum flavor to superposition is the fact of measuring the system. So I will illustrate it with an example. Imagine you have a system of arrows and you prepare the system so that all the arrows are pointing left. And then you go and you ask the system the question, are you up or down? And you ask every arrow. So because the arrows pointing left can be expressed or can be described as a linear superposition between up and down, you will get outcomes that are 50% up and 50% down when you ask the question, are you up or down? You get that result from the system and then you can prepare with that a different state of that system where half of the arrows are pointing up and half of the arrows are pointing down and you ask the same question. And you get the same result. There are two very different systems and you have got the same result for a measurement. Now how can you determine what the difference is? The systems are very different because in the first case you have prepared the system in a pure state. There is a question you can ask to the system that will give you a deterministic, not random outcome. And that question is, are you left or right? It's going to tell you 100% they are left. In the second case, there is no question you can ask. That will give you a 100% deterministic result because it's a mixture. So if you ask left or right, there's going to be a random outcome because half of them are up, half of them are down. If you ask up or down, same thing. It's going to be 50%. How does this relate to quantum random? So you can imagine that being similar to flipping a coin is going to be heads or tails. But it's not like flipping a coin at all because when you flip a coin, the only reason you don't know the result you're going to get is because you don't know the exact energy with which you flip the coin, the angular momentum, the ambient conditions in the room, and so on and so forth. So if you knew all those variables, you could predict heads or tails. There are variables that are hidden to the observer. And for a lot of years, scientists thought that there was similar variables in quantum mechanics that would reveal themselves as randomness. And it wasn't until the seminal work of John Bell in the 60s that actually it was demonstrated that that's not the case. And now every physicist acknowledges that there is no hidden variable theory that is compatible with quantum mechanics. So it is really true that when you measure a system, nature reveals randomness. And it doesn't matter how long I think about that. I will always find that amazing.