Letter Sounds

Fiona picked up Dr. Sears' Vaccine book.

Fi:  Mama's book.  (As she's looking at the picture on the front and points at the words below) This says boy.
Me: Buh, buh.  What letter says "buh"?  What  does "boy" start with?
Fi:  "B!"
Me:  Good job!  What letter says, "muh"?
Fi: Rhinocerous

Guess we need to work on that...