How Did She Know That?

As unschoolers, we don't do any formal teaching, so I am often surprised when the girls know certain things.  Sometimes they will know some scientific fact that they read in a book, or some piece of knowledge they picked up from watching TV.  As an unschooling parent you have to trust that when your child needs to know something they will learn it.  By coming to you for help or finding the knowledge on their own.  But it is still surprising sometimes.  An example would be the girls learning to read.  Lyric and Story both learned to read on their own.  I'm not saying Devon and I had nothing to do with it, but there was no formal teaching.  The same goes with math- we learn math by living and using it.
The other night Devon and I were sitting at the table and Story walked in.  This is what the conversation looked like:
Story--Mama, I know what 24+24 is. 
Me-- You do.
Story-- Yes, 48.
Me-- Yep.
Devon-- What about 32+32?
Story-- 64
Devon-- Okay, Story I'm going to see if you can blow my mind.  What about 35+35?
She sat there for a second thing...
Story-- 40
Devon-- No, 35+35
Story-- ........70

My question is, how did she know that?  I thought she was going to say 60 because maybe she was adding it in her head and she wouldn't know how to carry. 
It is so interesting watching the girls learn this way.  I am so thankful we stumbled onto this way of learning!