Why do people cry when they are sad, and when they are happy?

Hey, Matt:

Why do people cry when they're sad? I can understand tears when you've got something in your eye. But crying when you're sad doesn't seem to have any biological purpose.

-- Boo-hoo, the net

Matthew:

I've always wondered why people cry when they're happy. I'm sure if the Publisher's Clearinghouse people showed up at my door with a check for $10 million, I'd be crying all over the place.

-- Ruth, San Diego

When it comes to crying, we've got an expert right here. Nobody boo-hoos like Grandma Alice, and there's nothing like a field trip to bring science to life. If all water sources were as reliable as Grandma, California would be a rainforest. I think she's in the kitchen.

Hi, Grandma. You have company.

"Oh, Matthew, what's all this? Who are these people? We don't have enough chairs for everybody. I hope you're not expecting me to make coffee. Oh, Matthew, they're tracking mud on my nice clean floor."

Hey, no problem, Grandma. We're just going to give a little demonstration. Okay, move forward, people. Can everybody see? Short people can stand on the counter. Maybe it'll help if Grandma sits on the dishwasher. Up you go, Grandma.

"Matthew! What..."

Squeeze in, people. Plenty of room. Uh-oh, what crashed? Well, don't worry. We'll clean it up later.

"That was spaghetti sauce. For dinner tonight. Oh, Matthew..."

Okay, people. There are three kinds of tears, and chemically they're all a little different. We have about 20 tear glands that contribute certain chemicals to the mix. The first kind of tears constantly washes over your eyeballs to keep them clean and moist. The second type is what you get when you chop an onion, break your arm or get dirt in your eye or have your ears pierced. Here, Grandma, let me demonstrate.

"What? You're not going to poke me in the eye, are you? My ears are already pierced. Matthew!"

Naw, Grandma. I'll just pinch you real hard on the back of your arm a few times.

"Ow! Ow! Ow! Ow! Ow!"

See, people? See how we're starting to get a few tears? Show everybody, Grandma. These will be higher in bacteria-fighting proteins.

"Ow! Ow! Ow! Ow! Ow! Oh...Matthew, who's that man snooping through the freezer? He's eating ice cream and trying to hide a pizza under his jacket. Matthew, stop him right this minute!"

Sure, Grandma, I'll take care of it. So, people, we come to the question at hand. The third type -- emotional tears. The Steel Magnolias, Brian's Song, my-girlfriend-dumped-me kind. You saw Steel Magnolias, didn't you, Grandma?

"Oh, Matthew, you know what happens when I think about that movie."

Yeah, I sure do. Remember that sad part where the girl died?

"Matthew, I think that lady's stealing my good salad plates. Would you please get these people out of here?"

That really, really sad part where everybody's crying about the girl?

"Matthewwww..."

Think hard, Grandma.

"Ooooh, boo-hoo-hoo-hoo! Wawh! Waaaawh! Boo-hooooooo..."

All right! Here we go, people. If I collected some of Grandma's tears right now and analyzed them chemically, we'd find high levels of hormones that build up in your body in times of stress, ACTH and prolactin. And our old friend, the mood-altering, pain-killing endorphins. Scientists have found 25 percent higher levels of these proteins in emotional tears. Grandma, can you hold the sobbing down a little so they can hear me in the back?

So anyway, lachrymal glands have the ability to filter these things out of our blood, which leads some scientists to believe that crying is a physiological adaptation to stress. Certain of these chemicals are excreted only in tears. And stress can be caused by any major life change, good or bad, so winning ten mil would be, like, a shock to anybody's system. Here, Grandma, blow your nose on this dish towel.

"Matthew, please get these people out of here."

Psychologists believe crying might also be a social signal that encourages others to comfort us or protect us in some way. Of course, we've put all kinds of taboos on crying, so busting out in tears in the boss's office might not be a good idea these days, but that's just stereotyping of criers as weak or out of control.

"Matthew, get out of my kitchen, and take these people with you."

That prolactin stuff I just mentioned is a hormone that plays a part in the physical maturation of girls and in lactation, so here's one key to the speculation about why women cry about four times more frequently than men do. Men's emotional brains are wired up differently, too. Naturally high prolactin levels may make an individual more prone to weeping at the drop of a hat.

"And you're going to be crying four times harder than I did if you don't get out of here. Now!"

Just a sec, Grandma. So --

"Eeeek! What's that I smell? Smoke! Something's on fire! Matthew! Go! Boo-hoo-hoo-hoooooo...."