Best Friends and Alien Sounds

As I help out the grade 3 strings, I know what they're doing in class and what their goals are.  One of the pieces for the end-of-year concert is Lucy Locket, a piece they all know from classroom music last year.  It uses the sol-fa so, la and mi.  So to la is a step (they are only aware of major 2nds, or tones, at this point), and so to mi is a skip.  The class strings teacher has them starting with (for violin) 3rd finger on D (G) for so.  With this particular child, we'd played it in her lesson just like in class.  Then we got creative, and played it starting on open A.

Me: Now we're going to go back to starting on 3.  Instead of doing open A for la, though, what do you think we could do instead?
Girl: Ummm... could we use... [face scrunched up like this answer couldn't possibly be correct] 4th finger on the ... D string...?
Me [thrilled she worked this out without any prodding]: Yes!  Now, you know that 1 and 2 are not great friends... [girl nods] and 2 and 3 are best friends [girl nods] - what do you think about 3 and 4?  Will they be best friends or need a gap?
Girl [knowingly]: They'll need a gap.  You can only have one best friend.
Me: Good point.  You know another way we can tell?  If we put out 4th finger right next to our 3rd finger, it would make Lucy Locket sound like this: [I sing the beginning with a very squished up so and la]
Girl [laughing like this is the stupidest thing she's heard all day]: That's terrible!  It sounds like alien sounds!

I am pleased to report that her Lucy Locket sounded right, right from the start.