Screaming. Why God, Why?

Screaming. What about it? What's the big deal? It's a kid thing right? You hear kids at the park screaming their fool heads off all the time.
Well, my 18 month old daughter was born with something to say, and by God, we're all going to hear it.
She started at about 6 months, and from that point on, from first thing in the morning to when she goes to bed at night, we hear that shrill little voice screaming on the top of her lungs. When she was an infant, it was all the time for no reason at all, that doesn't happen so much anymore... thank you. But her older brother knows what buttons to push on her to really bring out the shriek. And then he starts too, and he's usually our quiet one.

Now on to the different types of screams, and this includes both of them for those who think I'm just going to pick on my daughter all day.
 The playing shriek A shrill, high pitched shriek that ends with equally high pitched laughter. Usually brought on by chasing one another through the house. I don't know what to do with this one. At least they're interacting in a positive way, which I want, right? Sibling best friends, playing and laughing and happiness. Until someone takes a header into the dresser... which leads us to....
The hurt shriek A full bodied scream to strike fear in any parent's heart. Especially since theres no audible difference between a stubbed toe and a severed limb and blood. This is the one that brings you in from the kitchen, dripping soapy water, to check the injured from head to foot. You don't want this to happen while you're cleaning out a chicken, because you can't touch a baby with unwashed chicken hands. Thats all they need, a split lip and salmonella. This is a toughie, because it sounds an awful lot like...
The pissed off shriek "He took my doll!" "She messed up my line of cars!" Because they aren't articulate enough to word this, I get shrieking. The shrieks are so hair raising and piercing, I'm more likely to just take the object in question away than solve any problems for the little monsters. Bad parenting? Or lesson learned? I don't know, because all I get in response is...
The "I want" shriek Again, I think or am hoping this is one that will disappear when they have better vocabularies, or learn to make their own juice cups. Maybe I'm kidding myself... but if I'm not, what about...
The just 'cause shriek Pretty self explanatory, no reason, just screaming. Usually in short bursts as she walks down the hall or as he sits in the highchair munching spaghetti. Which after an entire day filled with a myriad of screaming, shrieking and screeching, leads us to...
The Mommy shriek Yes, my friends, I scream at my kids, at the top of my lungs, sometimes. Granted, it usually takes me all day long of listening to all the other screaming... or maybe just something really out there. Yesterday I heard the familiar sound of raspberry-blowing... you know, that "Bpppppttt!" you make with your tongue. No big deal, my kids are 18 months and three, it's a favorite activity in my house for children and adults alike. But when I came out from the kitchen, I found out what that noise really was. Tyler was sipping up his sippy cup and then spitting it's contents into his sisters face. She was dripping and sputtering and gasping for breath! I was furious! At least it was only water! (Thank you mom for suggesting I cut back on the juice) He was SPITTING on her! What in the Wide World of Sports possessed him to do such a raunchy, nasty, UNSANITARY thing? And that's what I asked him on the top of my lungs. Have I mentioned I'm slightly OCD and one of my big things is saliva and people's mouths? I doubt he heard me though, like I always tell the kids.
"I can't hear you unless you use your quiet voice."
And I think thats what he responded with. I'm not sure, I was shrieking too loudly.