How should I tell a professor the answer to something he doesn't know? Last year, I asked my physics professor a question that he did not know the answer to. This year, while doing a research paper, I also figured out the answer to the question I asked him. I want to send an email to my professor explaining the answer I found simply because he's a curious guy.

Typically, if I asked something that the professor didn't know, I'd email them a few days later saying:


  Hi Dr. Professor, after some reading, I figured out the answer to why so-and-so happens. Here's what I learned.


But since it's been so long, he definitely doesn't remember what I asked him. So I'd like to remind him what my question was. However, writing:


  "Hi Dr. Professor, last year I asked you a question that you did not know the answer to, and this year, I figured it out. Here's what the question was, and here's what I learned"


sounds a bit insulting to me, because I'm writing that he didn't know the answer. What is a good way to share an answer with a professor without being "insulting"? Am I just overthinking this?
I think there is an easy way to phrase this to make it tactful, after all professors are often curious about learning new things too! 


  "Hello Professor X, 
  I hope you've been well! Last year I asked you a question and we couldn't figure it out at the time but I've since come across an interesting answer and just wanted to pass along the info just in case you're curious. We were discussing Y ... "