Expanding Vocabulary

Mia is turning into quite a talker. Nonstop. Most of it pretty intelligible, if one is sensitive to the context. I haven't counted, but I'm quite certain her vocabulary is over 500 words now, maybe closer to 1000. We've been really surprised at how she now strings 4-5 words together. A few hundred of her words she accompanies with ASL signs. She is really partial to the signs for animals and feelings.
Her favorite toy is without a doubt her magnetic letters and board. Followed by her Leapfrog spelling and phonics toys. She talks about letters all day long. She knows the name of every letter, and she is starting to learn the sounds they make. She asks us to spell words for her with her magnetic letters. Yesterday, she instructed me to spell "Barney" but I couldn't find the B or R. I said, "I'm sorry, I can't spell Barney. The B and R must be upstairs." To which she replied, "Find the letters upstairs." Maybe this isn't astonishing to you, dear reader, but as a relatively new parent I'm often stunned to see how Mia will struggle with a new skill, then have a big breakthrough followed by rapid learning.
We keep one of those magna-doodle tablets in the car, and to amuse Mia I'll ask take the stylus and ask her what she wants me to draw. She used to ask for shapes; now she also asks me to "Spell Mia. Spell Mama. Spell Daddy." Today for some reason she asked me to spell Tail. Or was that Tale? This is when I realize how indefensible the English language is.