Haha, well the instances of Bell's experiment that I've seen so far all refer to entanglement. It's pretty easy to think about some unknown quantity having a pr

Haha, well the instances of Bell's experiment that I've seen so far all refer to entanglement. It's pretty easy to think about some unknown quantity having a probability distribution on its states. If you have an electron, and you don't know the spin yet, you say that it can be either up or down. What's special about entanglement between 2 particles is that it says the distributions on the two are not independent. In the classic example, one can only be spin up if the other is spin down - so they have a statistical dependence on each other. Bell's experiment tells us that the strength of this statistical dependence depends on how you measure the particles, which isn't determined until measurement time, when the particles are far away. But that being said, you could imaging all other sorts of spooky action at a distance - like 2 particles that have never been local to one another having some sort of statistical dependence. But that would be really hard to show until you found what type of nonlocal statistical dependence to look for, so usually when we're trying to figure out a good model we end up thinking about entanglement.