I said: Yes, what is this?
My boy: It's a bus!!! The wheel on the bus...
What's his motive of doing so? It seems he get some amusements from hearing our replies and watching our reactions... ya, until we get annoyed (not so unless we rush on something) . Then, I tried to switch the scenario when he questioned one thing again. I decided to be passive in attending his questions. An example like the above conversation between my boy and I. It worked.
At such stage, he can speak and understand more things and therefore he feels excited to "test" his level of knowledge. On the other hand, he is very curious to learn more new things and hence his eager of questioning grows stronger. Also, by questioning, he gets attention from us too. That's my conclusion.
In his game of questions, I just wait for him to give the answer if I know he can answer it.