LIKE millions, perhaps even billions, of people, David Schmidt of Amherst, Mass., takes a shower every morning. For the last few years, between first spritz and final drain, a question has vexed him: why does the shower curtain suck in?

Today, thanks to $28,000 worth of high-powered computer software, a Ph.D. in engineering and too much free time, Dr. Schmidt, 31, a mechanical engineering professor at the University of Massachusetts, believes he has an answer. It has to do with why airplanes fly, hurricanes twist and apples fall.

True, the mystery of the sucking shower curtain does not rank high on the ladder of mankind's challenges, and he himself never encountered a sucking curtain until he showered one day at his mother-in-law's house. There, with the showerhead hissing away, the corner of her gossamer-thin shower curtain billowed in and clung clammily to his leg. ''It sucked beautifully,'' he recalled.

For years, apparently, engineering cognoscenti and amateur scientists have wrestled with clinging shower curtains. In 1994, Mr. Schmidt encountered the seemingly humdrum problem on his doctoral examinations at the University of Wisconsin. One camp favors something called the Bernoulli principle, which holds that as water, air and other fluids accelerate, their pressure drops, leading to lift. Like an airplane wing, the shower curtain moves, the Bernoulli backers say, because water from the showerhead accelerates air on one side, letting air rush in and move the curtain.