Are words thoughts?

I don't think so, although words, like other signs, can be associated with thoughts, but I don't think the words and thoughts are interchangeable. I can think about words, but I don't think I think in words or sentences. I think in thoughts which are, in some way, related to words.

Is thinking merely talking to yourself?

Not anymore than humming to yourself is thinking. Are thoughts tangible? How do I model a world, by humming, speaking, or thinking? I think that seeing is closer to thinking than speaking is. Whatever it is that maps thinking to speaking it is fundamentally different from what maps feeling to making music. Except, of course, that much of our thinking is abstract. How do we visualize the abstract? By making it concrete. How does a preposition mean? Is it the same kind of meaning / expressing as a noun or a verb?

Ceci n'est pas un seing.