The Hummingbirds Puns

My father was playing with my two year old son (his grandson) in the yard and was talking to him about birds when they spotted a hummingbird. They were a little ways away from us and didnt even realize that I could hear them talking. My father says... "do you know hummingbirds hum?" to my two year old. Of course my two year, who is just learning to talk, didn't really have a reply. My father then says "CAUSE THEY DONT KNOW THE WORDS". Then he laughed and laughed while my two year old stared at him. Groans could be heard from across the yard.