C'est exactement ce à quoi je pense !

"Ce à quoi" is use to point something, like "what I mean", "what he does", "what they're thinking" . Today, lot's of people use "ce dont" or "de quoi" in a similar way. Like "ce dont-il parle" ("what he's talking about").

Ce à quoi + sujet + penser, c'est.... Ce à quoi tu penses, c'est (à) sa santé = What you're thinking about is his health. Tu penses à sa santé = You're thinking about his health Using "Ce à quoi" means you stress the topic of thought, rather than the fact you are thinking.

In French "quoi" alone would not work in that sentence (even though in English it would). It would make the sentence sound like a question. To avoid that, by adding "ce à quoi" it becomes sort of "that of which he's thinking". - à quoi penses-tu? (notice that even in the question, there is this "weird" but necessary à) - Les vacances, la mer... oui, c'est ce à quoi je pense.... - Non, ce à quoi tu penses c'est le bar à volonté sur la plage !