Computer scientist Luis von Ahn created the games as a way to solve problems that are difficult for computers, even though the problems may seem simple to us. Computers, for example, have a hard time identifying a cat in a photo, while we can spot a cat at a glance.

Most people would have no trouble identifying the kitten and two puppies in this photo—a task that a computer can’t yet accomplish.

The idea is to invent online games that people enjoy playing. Then, as people play, they provide data that researchers can use to improve computers. The players don’t know that they’re helping out. They’re just having fun.

“The potential is huge,” says computer scientist Manuel Blum, one of von Ahn’s colleagues at Carnegie Mellon University in Pittsburgh. “These ‘games with a purpose’ make use of humans in a wonderful way.”

By harnessing the brainpower of thousands of people playing games on the Internet, computers may learn how to identify pictures, translate Web pages into forms that blind people can use, develop common sense, understand foreign languages, and more.

Seeing things

Our brains are wired to collect and process lots of information about what we see, von Ahn says.

Computers, on the other hand, simply “see” a grid of dark and light dots, called pixels. It’s not obvious to a machine that a certain blob is a cat. Likewise a computer doesn’t know that a tiger, a cartoon feline, and a kitty curled up on a couch all belong in the same category: cat.

“If you give me an image of some common animal, I can tell if you if it’s a cat or a dog,” von Ahn says. “Computers can’t do that yet.”

Because computers can’t make such distinctions, search engines have a hard time finding images on the Internet. And getting people to label each image doesn’t work well. There are billions of images, and the job is pretty boring.

The ESP Game (see www.espgame.org), invented by von Ahn and his coworkers, provides a quick, fun way to label images.

The ESP Game has collected more than 20 million labels for images since it debuted in late 2003.

When you play the ESP Game, you get randomly paired with another player. Both of you see the same image on the screen, but you can’t communicate in any way. You type in a word that describes the image. As soon as your partner types in exactly the same word, you both earn some points and another image appears. You can keep typing in new words until you again get a match, or you can pass and go on to the next image. Each round lasts 180 seconds and includes up to 15 images.

The first time I played the ESP Game, my Internet partner and I had a score that was better than the scores earned by only 40 percent of people who had played before us. We did better on the next round, though. And the next. It was hard to stop.

Although I should have been working, I played without feeling guilty. I knew I was doing something useful. Every match became a label for an image.

After each round of the ESP Game, you and your partner can find out where you stand.

So far, von Ahn’s game program has collected more than 20 million descriptions. And the game appears to do its intended job.

“The labels given by the ESP Game are as accurate as ones generated by someone paid to label images,” von Ahn says. Players also come up with about the same number of words for an image as paid professionals do, he adds.

Google has now developed Google Image Labeler (see images.google.com/imagelabeler), its own version of the ESP Game.

Where’s Waldo?

The labels provided by the ESP Game tell you what’s in an image. But the game doesn’t tell you where in an image a certain object might be.

To help solve this problem, von Ahn and his coworkers invented the two-player game Peekaboom (www.peekaboom.org).

In Peekaboom, one player tries to guess a word as another player reveals more and more of an image.

One player sees an image—say, a cat playing with a dog—along with a word related to the picture. The other player sees a blank screen. By clicking on the image, the first player reveals a small piece of the image. The goal is to get the second player to guess the word in as few clicks as possible. After the second player correctly guesses the word or passes, the two players trade places. Each round lasts 4 minutes.

With each correct guess, the computer collects data about where things are within a picture. “That’s exactly the type of information needed to train computers to see,” von Ahn says.

Another image-labeling game, called Phetch (www.peekaboom.org/phetch), goes even farther. Instead of entering a single word, the first player writes a description of an image. Given that description, three other players then race to pick the right image out of a million or more possibilities. The program collects the descriptions that lead to quick results.

The game Phetch is an online treasure hunt that matches descriptions with images.

Programmed with such descriptions, computers might eventually be able to describe images to people who can’t see. Software already exists that can read aloud words on computer screens, but these “screen readers” can’t yet explain pictures.

Having fun

As useful as these games are, they won’t work unless people play them. That means the games must be fun. And figuring out what’s going to be fun is easier said than done.

Like game manufacturers, von Ahn tests his games on people. On the basis of how long people play, what they say about the games, and which ones they choose to play again, von Ahn makes changes until he comes up with something that works.

“Nobody knows what makes games fun,” von Ahn says, just like there aren’t any rules for what makes a movie popular. “There’s no formula,” he says. “It’s still an art.”

Still, von Ahn has noticed some patterns. Randomness and unpredictability help, he says.

Players like to be able to get strategy tips that improve performance, Blum adds. Large point bonuses can’t hurt. People like games that are competitive too. Displaying lists of top scores helps keep people playing.

Do you have thoughts about what makes a game great? Send them in, and we’ll share them with the researchers. Your ideas could become part of a future formula for fun.

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And wherever there are computers, there have to be computer programmers who write the instructions that tell the computers what to do, step by painstaking step.

Want to learn how to program a computer and have fun at the same time? Give Alice a try!

Stage3 Research Group, Carnegie Mellon University

The skills of programmers, computer scientists, and software engineers are essential not only for generating special effects in movies but also for creating video games, operating cell phones, searching the Internet, downloading and playing music, and much more.

Nowadays, there are computers in airplanes, cars, television sets, vending machines, and kitchen appliances. All these computers need people who understand how computers work and can provide the necessary instructions. A set of instructions for a computer is called a program.

Unfortunately, the popularity of computer science as a career has been fading in the United States. There are 50 percent fewer computer science majors at U.S. universities now than there were 5 years ago, says Randy Pausch. He’s a computer scientist at Carnegie Mellon University (CMU) in Pittsburgh.

Equally troubling is that fewer than 20 percent of these students are women, he adds.

“Computer science has tremendous power to make a big difference in people’s lives,” says Caitlin Kelleher, a graduate student in computer science at CMU.

Kelleher wants middle schoolers, especially girls, to get excited about working with computers. Using an interactive computer program called Alice, Kelleher has made it her mission to show kids all the exciting opportunities that open up when you can talk to these machines in a language that they can understand.

Using Alice, a student can quickly create an animated movie in which characters move about and interact in an imagined 3D world. Along the way, the student learns how to write a simple computer program.

The results don’t look like Lord of the Rings, but if you give Alice a try, you can experience the immense satisfaction of getting a computer to do what you want it to do—and of showing off your waddling penguins or attacking bugs to friends.

Action commands

In a computer program, each instruction specifies an action. Writing a program to animate 3D objects is all about deciding what actions you want these objects to perform.

With Alice, students begin by crafting stories. Then, they work out lists of actions that must go into the programs to tell the story. The students learn how to break a large problem into smaller pieces. It’s a bit like doing a word problem in math.

Users select characters, such as ice skaters or monsters, and environments, such as a forest or a city. They then create scenes in which the characters talk and move around in these environments. The results are satisfying little animations that can be funny, sad, or even weird.

Working with Alice, students aren’t faced with the hardest part of writing a computer program—specifying in excruciating detail, and in exactly the right language, every little thing that has to happen.

For example, suppose you had to tell someone who had never been in a kitchen how to make a peanut butter-and-jelly sandwich. You couldn’t just say, “Make a peanut butter- and-jelly sandwich.” You’d have to specify where to find the ingredients and the knife, how to use the knife, how much jelly and peanut butter to measure out, how to assemble the sandwich, and so on. You’d also have to tell the person what to do if something went wrong—for example, if the jelly jar were empty or there were only one slice of bread.

“If you write a paragraph in English and you put a semicolon or comma in the wrong place, people will still understand what you’re talking about,” Kelleher says. “With computer languages, if you get one thing wrong, it doesn’t work anymore.”

With traditional programming, beginners often get frustrated. They’re unable to figure out what they’ve done wrong.

With Alice, students use a mouse to select commands. Then, they can watch what happens on the screen. If a monster happens to end up in the wrong place, for instance, they can simply change the command.

Improved grades

So far, Alice seems to be doing its job. It’s getting students more interested in computers.

One study looked at college freshmen who were interested in computer science but were at risk of failing because of low grades. Without Alice, Pausch says, the students averaged a C in their first computer science class. Only 47 percent went on to take another computer class.

The average grade of students who used Alice, however, jumped to a B. Eighty-eight percent of these students took a second class. Alice conveyed the basic ideas and purposes of programming. After that, students were better able to learn the specifics of computer languages, such as Java and C++.

Teachers at 50 colleges and 40 high schools are now using Alice to teach introductory computer science classes, Kelleher says. She has been using a modified version with Girl Scout troops.

In Colorado, graduate student Agata Dean and her coworkers have used Alice at the middle school level in an after-school technology club and in a weeklong technology summer camp at the Colorado School of Mines. In both cases, students learned the software and were able to begin using it quickly, Dean says.

The beauty of Alice, Pausch says, is that it shows students how exciting and creative computer science can be. Working with computers doesn’t make you a geek, he insists. Computer work is an art form, only better.

“Painters are limited by paint, and sculptors are limited by clay,” he says. “Computer programming is limited only by the limits of the imagination . . . . There’s nothing you can’t do.”

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Word Find: Alice