I said, 'Hey, what's so funny?'

One Friday afternoon in an elevator filled with silent professionals in a Manhattan office building, I burst out laughing. What set me off was thinking of a story about a duck that a friend had told me a few hours earlier. I tried to think sobering thoughts and to force my face muscles into a serious expression.

It didn't work. I kept thinking about that duck. My shoulders were shaking, and when I tried to keep the giggling noises from coming out my mouth, I snorted through my nose. My fellow passengers continued to be silent and straight-faced. They politely looked at the elevator buttons or at the crack between the closed doors, instead of at me. I was embarrassed by my lack of self-control, but I couldn't stop laughing. I'd had a hard week, and a brief period of helpless laughter, frankly, made me feel better.

Laughing, especially outright hysteria, always does me a world of good. And it doesn't even have to be me who's doing the laughing. I can get an equal sense of pleasure by rendering someone else helpless with laughter.

My own mother is a perfect subject. Sometimes when my sisters and brother and I are all home - for Christmas or some other family occasion - we work together. We don't plan it or even talk about it later; it's just something we all know how to do. It can start with a single, humorous remark at the breakfast table that makes our mother laugh loud and long.

Next, before she's quite caught her breath, someone else might create a sillier variation on the original joke. At this point, our mother might place both hands on the table and lean forward slightly as she laughs harder. If we are all attuned to the opportunity presenting itself, one of us will get up from the table and try an impersonation, still sticking with our main theme. Then, if it's a good enough imitation, our mother will push her chair back from the table , cross her arms over her stomach and make no sound at all, except for the occasional prolonged ''ha'' and gasp for breath. Then the impersonator, spurred on by the strength of the response, will go to greater lengths, making gestures that are even more ridiculous and jokes that are connected to reality by only the slimmest thread.

The goal we are all trying to achieve takes a real pulling together. Someone has to play straight person, feeding lines to the impersonator. A third has to listen for the chance to throw in the occasional joke of his or her own so that the pace never slows. The fourth has the most essential task of all: to heighten our mother's hilarity by laughing with her. The more laughter there is, the funnier things seem.

When all goes well, this is what happens: She laughs so hard she has to wipe tears from her face; her lips are stretched open, tight across her teeth; she raises one weakened arm in protest and manages to squeak out the word, ''Stop.'' That's when we give it one more push. By now, no more jokes are necessary. All we need are a few straight-faced remarks like ''Gee, Mom, it wasn't that funny.'' Then we see her go over it once again in her mind and decide that yes, it was that funny, and laugh even harder, remembering. It all ends with a gradual wind-down, smiling pauses that grow longer and longer between bursts of laughter, mere echoes of earlier moments.

This week I was riding in a cab in which the driver was drinking from a paper cup and listening to the news on the radio. All of a sudden, he made a sound that was between a cough and an exclamation and spilled his drink all over his shirt. He was laughing too hard to mind the mess. I looked around to see if there were anything funny happening near us on the street, but there wasn't. I tried to remember whether the newscaster had just said anything comical. Not at all. The driver was apparently just laughing at something he'd thought of. I said, ''Hey, what's so funny?'' But he couldn't tell me. Every time he tried, he laughed so hard he couldn't get the words out.

Finally he waved his hand in a gesture meaning that whatever he was laughing at was too silly to repeat anyway.

By the time he'd taken me where I wanted to go, he'd recovered almost entirely. I paid him, thanked him, and, as I got out of the cab, said, ''And no more laughing.'' That was all he needed to start all over again. I left him hunched over the wheel, clutching it with both hands, eyes shut tight, leaning slightly to one side, saying, ''Hee ... hee ... hee....''