Me and my buddies were debating this last night. I was always taught that the piano is a percussion instrument, and that it is often mis-labeled as a string instrument because of it's internal strings. The argument being that mallets hammer the strings to create the sound, and the harder or softer you strike the strings changes the sound.
My buddy, who is a piano player, said it's a string instrument. I tried looking it up online and all I found was an extensive debate on this issue with no final consensus.
That's when I said: "To the forum"
So which is it? I still vote percussion because there's no other way to play a piano other than striking the strings, as opposed to the guitar or violin or cello where you you can pick the strings or use a bow.....basically your fingers have direct contact with the strings on those instruments, instead of an intermediate mechanism that strikes the strings.
Edit: sorry if this thread has been posted before. I tried searching and didn't find anything on the initial results.