A talking cake

Last year in May, my friend from Stockholm sent a MMS to me. It was a birthday-greeting from him to me, a picture of a birthday cake with the text "Happy Birthday to you!" (in Swedish of course). My friend found the cake on three:s homepage and sent it to me. That was a funny cake.
Later in the summer, my sister had her birthday in July. I sent the birthday-cake as MMS to her, with same text and congratulated her on her birthday.

Some minutes later, I got a text message from her, saying "Happy Birthday on your 39-years day? I am only 36 years!" What? I haven't written that! Where did she get that from?

My sister explained to me, it was the birthday-cake that talked and said that. Oops! I didn't know that the cake talked too! Because I'm deaf, I didn't heard that.

But my sister understood that I couldn't hear what the cake said.
She was happy anyway. :-D