a few days ago:
Ethan: (listening to radio) Hey, I have that song on one of my CDs, except it's in a different key.
me: What key is this in?
Ethan: It's in G.
me: What key is the other one in?
(He hasn't ever tried to play the song on the piano or use any other method to figure out the key.)
me: Hey, aren't you learning to play this song in your piano lessons?
me: Is it in the same key?
me: What key do you play it in?
me: What key is this version in?
I didn't check to see if he was right. He might have trouble with sharps and flats, and he may not be precise yet with it, but this is well beyond most musicians even after lots of training. So we can add this to the ability to drum a 7/8 rhythm at age 6.