Do humans entangle particles, or are they already entangled? I recently visited our city hall and there was an exhibition which featured one of our local physicists, a John Stewart Bell. Although it was presented well the technical information was very basic. I hit the internet after to try and figure out some of the terms but found many of the explanations either sensational or just plain opaque (to someone without maths and physics degree). Even the "Entanglement for Dummies" type pages were confusing though I have no way of telling if the writers were careless or just beyond stooping to explain to laymen. In any case I'd like to ask a few simple questions here. I know this is probably a professional website and many of you use it for work and research but John is a local source of pride it seems and I should really have some basic understanding at least. So here goes...
1) How can humans entangle particles or are they already entangled?
2) When you measure something about an entangled particle why does its partner react?
 A: 
How can humans entangle particles or are they already entangled?

Both. You can think of "entanglement" as just a particular type of state in which particles (or more generally physical systems) can find themselves in. It has a bunch of properties we like, but it's still a fairly "natural" type of physical state which. If you were able to measure things with a lot of precision, you probably find that virtually everything around you (at the atomic scale) is "entangled" with something else.
We can also generate entangled states in laboratories. Exactly how you do it strongly depends on the physical system. If you want to generate entangled photons you would use special types of semi-reflective mirrors and/or special types of crystal which react in a particular way with laser light passing through them. Entangling charged particles will usually involve some form of magnetic field, etc.

When you measure something about an entangled particle why does its partner react?

"Why" is a tricky question. We observe that it does. There is not, in general, a more "elementary" or "intuitive" explanation as to why it does. I guess one thing you could say is that "entanglement" is a form of correlation between particles (more precisely, between measurements performed on the particles), and the reason such a correlation exists is because the particles somehow interacted before. Think of two billiard balls: after they hit each other, their future positions are also correlated (if one goes left the other one goes right). Entangled states are sort of like this, except it's a much stronger form of correlation which cannot be explained (as per Bell's theorem) classically.
