Different nuances of そっか and なるほど？


I’ve learned that both mean something like “I see” or “I understand” but are
they interchangeable? Is there any difference?



  * そっか is very casual, and it's clearly inappropriate in formal settings. On the other hand, なるほど is safe in business exchanges, but it can sound a little pompous. If a small kid said なるほど, it would sound funny.
  * Both なるほど and そっか are used like "Aha, I (finally) got it!", but I feel そっか is mainly used when you have realized a bad thing, e.g., "Oooh, so I was wrong...".

