What is the first word the baby learned to say?

W: There are many interesting stories about how a baby learns to say its first word. Now, Mark, would you like to share your story with us?
M: OK, here is my story. I was about 18 months old, I think. And I had just started saying my first word. Every time a truck went by, I shouted a word that sounded like "truck". But to everyone else it sounded nothing like it. My parents tried to correct me, but failed. Then about 3 months later, we traveled to New York. As we were waiting for our plane in the airport, I looked out of a huge window. At that very moment, I saw the largest truck in the world. I had to tell someone about this. I turned my father, who was standing to next about a thousand people waiting for flight and screamed "Truck! Truck!" My father tried hard to shut me up, but I was so excited and I kept screaming my special word. My poor mother decided that she didn’t know this baby and ran to the lady’s room where she hid until I finally stopped. Well, I was talking like a normal child. However, my parents will never forget the very first word I pronounced in my life.
W: That is really a very interesting experience about learning to say the first word.
M: Yes, so every time I come to see my mother, she always talks about this to me.