I am really grateful to you for your explanation! I suppose that in Russia we have the same thing about words "think of" and "think about". But we don't change/add the prepositions. To show the difference between them we add prefix ("приставка") to a verb "think" ("думать" in Russia). So, we say "подумать" (prefix "по") in the way you've described using "think of". And we say "думать" to show the process (you say "think about').
"Think of" is like a "Look at", when we have only some object by itself. But if we are looking at a tornado and thinking ____ physical, flow, etc. processe inside it, we "are thinking of" or "thinking about"?
