My question is simple, although I suspect the answer may not be: What exactly causes frame-dragging? I'm not very adept at understanding GR equations, so you'll have to explain it in vernacular. I find the concept of frame-dragging very interesting. One could posit the question: "If a black hole is rotating, how would you know which way it's doing so?" A very naive understanding of black holes would answer that you don't. However, the correct answer is that you measure frame-dragging in the vicinity of the event horizon (which can theoretically be done simply by dropping an object towards the black hole and seeing which direction it starts to move as it falls.) (Another naive question would be "how can a point rotate? It has no dimensions, and thus can't possibly rotate." But the correct answer is that the singularity of a rotating black hole is not a point, but a ring, which is what "allows" it to rotate, so to speak.) The concept of the ergosphere, which is caused by frame-dragging, is even more interesting. It baffles the mind that distances within the ergosphere may increase faster than c from the point of view of an external observer... which just raises so many questions.