triding down a hallway at M.I.T., Amy Smith has a bucket in one hand and a length of string in the other. The brain behind such creations as the phase-change incubator, she is on her way to the Charles River to retrieve some dirty water for her next class. As she lopes along, Smith describes the ordeals of testing water in remote villages. Her words spurt out. She's a woman on fast-forward, and she does not so much talk as download information.

We reach the massive front doors of M.I.T.'s main building, and she pushes out into the crisp air and roar of traffic. Without interrupting her disquisition on water-testing, she perches sidesaddle on a banister. Elegantly poised, she slides down the handrail, still talking.

She lands on the sidewalk with a practiced leap. ''You slide faster in the winter,'' she says, ''when you've got a wool coat on.''

If you're an inventor and you ride the banister below the pillared entrance of a university, there's a good chance of coming off as way too cute, like Robin Williams playing an inventor in a Disney movie. But Smith can get away with this kind of flourish. She is, after all, one of the brightest minds in a movement that sets out to prove that the best technology can be cheap and simple. The banister is a perfect example: it requires less energy than the stairs, and it's free.

In a culture that hails mobile phones and plasma-screen televisions as the great innovations of our time, Smith is gloriously out of step. She designs medical devices and labor-saving machines for people who live at the far end of dirt roads in Africa. Her inventions cost anywhere from a few hundred dollars to a few pennies. ''You can't understand how important a grain mill is,'' she says, ''until you've spent three hours pounding grain and gotten a cup and a half of flour.'' It is this kind of understanding -- of tedium, of tired muscles, of hunger pangs -- that Smith brings to her work.

n hour later, in Smith's ''D-lab'' class, students gather around a huge, black-topped slab of a table. It's the first semester of design lab, and these undergrads are learning about the politics of delivering technology to poor nations, how to speak a little Creole and the nitty-gritty of mechanical engineering; during the midsemester break, they will travel to Haiti, Brazil or India. There, they will act as consultants in remote villages, helping locals solve technical problems. Oh, yes, and the students will also test village drinking water for dangerous bacteria.

Today, Smith is training them to do that. Using a small pump, the students draw the Charles River water through a filter. Smith points to a piece of the testing rig -- what looks like a silver barbell. ''This test stand costs $600,'' she says. ''Personally, I find that offensive.'' When the students work in the field, she says, they will be using a far cheaper setup -- one that she patched together herself for about $20, using a Playtex baby bottle. ''You can do a lot more testing for the same amount of money.''

Now the students have made cultures of Charles River water in petri dishes. The next step is to incubate the petri dishes for an entire day at a steady temperature. But how do you pull that off in a lean-to in Haiti, with no electricity for miles around? Again, Smith has a solution. She passes around a mesh bag of what appears to be white marbles. The ''marbles'' contain a chemical that, when heated and kept in an insulated environment, will stay at a steady 37 degrees Celsius for 24 hours. The balls are the crucial ingredient in one of Smith's inventions -- a phase-change incubator that requires no electricity. The design won her a 1999 Collegiate Inventors award. She says she hopes that the Centers for Disease Control and Prevention will soon endorse her incubator. ''From there it's not a big step to go to the Red Cross,'' she says. One day it could be a key piece of equipment at rural health clinics, where doctors depend on intermittent electricity or none at all. Smith has founded a company to handle the exigencies of getting her incubators up and running and out into the field. ''I have 6,000 of these balls on their way here from China as we speak,'' she says.