The more you know about a subject, the more interesting the dumb-sounding questions get.

Simple questions get deeper as your knowledge grows. Early on, asking why something happens has an answer somebody can tell you. Later, the same words lead straight to the parts nobody has settled, because you now know enough to see where the explanation thins out. That is why people who know a great deal ask childlike questions and mean something quite different by them.

Go back to a basic question in a subject you know reasonably well and try to answer it fully, out loud. Keep asking why until you run out. Where you stop is the edge of what you understand.