When I came back up , I heard enthusiastic WEE - OO - WEE - OO coming from his room . I popped my head in there and he made the noise at me and grinned . I asked " Firetruck ? "

Question: Why might he make that sound ?
Answer: I said " firetrucks " so he made the noise .He was playing make - believe with himself .I like firetrucks so he made the noise .He felt enthusiastic that I was in his room .