This morning, we were being silly and I said, "I'm a banana", and Amy said, "And I am a pear".
I asked her what Mummy was and she said, "An apple".
The significant thing about this is that so far, she's always used "a" as the indefinite article—but today, she used, correctly, "an" before a word starting with a vowel sound.
One small step for a girl, one giant leap for mankind! (Or something like that.)
Edit: OK, when I asked her half an hour later what Mummy was, she said, "A apple". But still!