IT'S pretty hard to miss Andrew's enthusiasm.The preschooler punches the computer keyboard. He looks raptly at the computer monitor. Whenever he gets a right answer, he raises both arms above his head in triumph. Cecile is quieter but more accurate. Her task is to put in order four cartoon pictures of a pig making an apple pie. She hits the space bar of the keyboard until the 1 is beside the picture of the pig picking apples. Then she presses the Enter key. "Professor Al a likable guy on the righthand side of the screen - gives a reassuring nod. Cecile then orders, correctly, pictures of the pig cutting the apples, baking the pie, and eating it. The computer lights up its approval, arcade-style. Professor Al takes a break with his yo-yo. For the preschoolers here at Carnegie Mellon University's day-care center, this is a typical activity. The computer revolution is not waiting to happen for them. It's already here. In another Carnegie Mellon building across the street, a class of four-year-olds is learning a new drawing program, called Kid Pix. "This program has surprises in it," says parent-volunteer Becky Shapiro. The idea: Let children discover its features on their own. Anne has discovered the eraser tool. She clicks on the eraser icon on the screen with her computer mouse, then pushes the mouse back and forth vigorously. Slowly, a Kid Pix picture is revealed. "It's an eye," she confides as the picture appears. These children may not be typical. Their parents are university faculty or staff. Still, what is happening here is taking place to one degree or another in schools across the United States, indeed around the world. There are some 3 million computers in US schools - or roughly one for every 13 students, estimates Henry Jay Becker, principal research scientist at the Johns Hopkins University Center for Social Organization of Schools in Baltimore. The number is growing steadily by 300,000 to 400,000 a year, he says. Other countries aren't far behind. According to Dr. Becker, a 20-nation survey in 1989 found that Canada's province of British Columbia had a greater concentration of computers at its high schools than do schools in the US; the Netherlands was roughly comparable; Japan, slightly behind. These countries did not have as many computers in elementary schools as the US does. But Israel was roughly comparable in terms of computers per school, although it lagged at the high school level. Ronghua Ouyang, a graduate student who is studying the children and their computers, has found that nearly all the preschoolers at the Carnegie Mellon Children's School learned quickly how to use the machine. In one semester, they went from needing a lot of help from their teachers to almost none. Typical of what's happening in other schools, the children begin to help each other. That's very evident at the Children's School computer lab. Students constantly lean over to see what their classmates are doing. "And they enjoy it," Mr. Ouyang says. As long as children get equal access to a computer at a young age, he says he believes that girls and boys will perform equally well on the machine. The potential changes in education are dramatic. But teachers and researchers are quick to point out that, at the elementary level at least, the changes will be evolutionary rather than revolutionary. "At the younger grades you would expect that human-guided activity is more important than mastery of the machine," Becker says. The teacher-student interaction will remain the primary mode of learning. "We see it as a tool," adds Marsha Poster, director of Carnegie Mellon's child-care center. "We don't see it as an end-all." Perhaps the best hints to the future come from the children themselves. Earlier this year, International Business Machines Corporation (IBM) sponsored a contest for Pittsburgh-area children asking: How will computers be used in the home in the year 2000? (The drawing on this page came from that contest.) Bernadette Giovanelli, a second-grader from Richeyville, Pa., drew a picture of "Compute-A-Sibling" - a computer aid for the only child. Jonathan Devore, a third-grader from Coraopolis, Pa., went wild with a picture of an IBM remote-control computer, which did everything from adjusting the height and width of the bed mattress to setting the temperature of the bath water. Pittsburgh first-grader Julia Toal drew a computer screen displaying a heart with the message: "What is your problem?" Underneath the picture she wrote: "In the year 2000 a computer might be able to solve problems." "We have taken a lot of things from kids' lives," says Ann Taylor, director of the Children's School. Urban children don't have the trees, the open spaces, and the farm animals that their grandparents grew up with. Maybe the computer - with its ready access to the outside world - will help give some of that back, she says.