Disclaimer: I don't now much about music theory.
Anyway, my wife just bought one of those, which I think is called a "tongue drum":
What strikes me is that, as bad as I am at playing music, whatever I randomly do on it, however I play it, it seems to make harmonious music. Or at least, much more harmonious than what I could randomly do on a piano or a guitar. My son, who is 5, plays with it too, and it just works. It seems this thing prevents anyone from playing dissonant music.
I played some other versions of this instument, which I'm pretty sure had a different number of notes, and probably organized differently, and the result was similar.
So, is there a particular method of selecting/organizing notes on an instrument which will make any sequence played with it sound harmonious? Is there a theory behind this?