Refuse to call Mummy

I wonder why, she can sweetly call Daddy and everybody else but when it comes to calling me, she will talk in gibberish. The other day, I told my Mum about it and she asked Tara to say Mummy and she said it nice and perfect. She made me feel like the boy that cry wolf.

To further convinced myself that she indeed refused to say Mummy, I deliberately said a whole list of names to call. And inserting "Mummy" randomly. She can call every single name except for Mummy. She can even pronounced the difficult ones like "奶奶", "舅舅", "姑姑". Even though 舅舅 sounds like "dou dou", at least she meant to say it proper. And how she call "Mummy"? Just gibberish.

I wonder why.