I knew it was piano right when I saw "felt-tipped hammer." If you open up a piano, you can see the little mallets striking the strings when you push down a key. And yes, a piano is percussion because you hit the keys, and it's also technically a string instument because the vibrations of the steel strings is what's making the sound.
(I've been playing piano for a decade or so, so please forgive my complete geekiness haha)