Originally Posted by
me
"Can you guess what I'm thinking of?" is similar, since "of" is tacked on by people who thought "Can you guess what I'm thinking?" didn't sound "right" to them.
Shinta was getting to the point with this one. The "correct" way to say it would be "Can you guess of what I am thinking?" or "Can you guess about what I am talking?" But no one says that, do they? Well, some do, but most people find it an odd construction. Anyway, with either variation the meaning is left intact.
I suppose the reason it's evolved as it has is because people tend to just leave the entire constituent intact. That being
"Thinking of [x]" where
[x] is the direct object. People don't like to break up that entire chunk.
In either variation the interrogative word "what" is meant to take the place of the
[x]. In the "correct" variation, "of" is moved to create a complete prepositional phrase
"of what". In the "incorrect" variation,
"Thinking of [x]" is left intact. However, neither is more logical, and neither is any more inherently correct than the other. I'd like to see the argument that says otherwise.
Usually it boils down to "because that's what I was taught in school."