Why do 1 year olds scream?

My little one year old, who is actually 20 months, has this horrible habit of screaming. For some reason there is no "real" meaning to his screams and they can be used interchangeably. In fact he will scream out of delight, if he is angry or frustrated. I used to yell "Why is the baby screaming." Only to get the same response every time echoed back to me "I don't know." Then I would go about accusing the other two kids of torturing their baby brother. I have since learned my lesson.... one year olds are smarter then they look. He is screaming cause of the response I was giving the other kids. He loved watching his older siblings get in trouble. Hmm.... now, how do I get him to stop screaming.... that is the question of the day.