"After the third or fourth season, I'd say it was rare that we ever had a segment below eighty-five percent. We would almost never see something in the fifty to sixty percent range, and if we did, we'd fix it. You know Darwin's terms about the survival of the fittest? We had a mechanism to identify the fittest and decide what should survive."

The most important thing that Palmer ever found out with the Distracter, though, came at the very beginning, before Sesame Street was even on the air. It was the summer of 1969 and we were a month and a half from air date," Lesser remembers. "We decided, let's go for broke. Let's produce five full shows — one hour each — before we go to air and we'll see what we've got." To test the shows, Palmer took them to Philadelphia and over the third week of July showed them to groups of preschoolers in sixty different homes throughout the city. It was a difficult period. Philadelphia was in the midst of a heat wave, which made the children who were supposed to watch the show restless and inattentive. In the same week, as well, Apollo 11 landed on the moon, and some children — understandably — seemed to prefer that historic moment to Sesame Street. Worst of all were the conclusions from Palmer's Distracter. "What we found," Lesser says, "almost destroyed us."

The problem was that when the show was originally conceived, the decision was made that all fantasy elements of the show be separated from the real elements. This was done at the insistence of many child psychologists, who felt that to mix fantasy and reality would be misleading to children. The Muppets, then, were only seen with other Muppets, and the scenes filmed on Sesame Street itself involved only real adults and children. What Palmer found out in Philadelphia, though, was that as soon as they switched to the street scenes, the kids lost all interest. "The street was supposed to be the glue," Lesser said. "We would always come back to the street. It pulled the show together. But it was just adults doing things and talking about stuff and the kids weren't interested. We were getting incredibly low attention levels. The kids were leaving the show. Levels would pop back up if the Muppets came back, but we couldn't afford to keep losing them like that." Lesser calls Palmer's results a "turning point in the history of Sesame Street. We knew that if we kept the street that way, the show was going to die. Everything was happening so fast. We had the testing in the summer, and we were going on the air in the fall. We had to figure out what to do."

Lesser decided to defy the opinion of his scientific advisers. "We decided to write a letter to all the other developmental psychologists and say, we know how you guys think about mixing fantasy and reality. But we're going to do it anyway. If we don't, we'll be dead in the water." So the producers went back and re-shot all of the street scenes. Henson and his coworkers created puppets that could walk and talk with the adults of the show and could live alongside them on the street. "That's when Big Bird and Oscar the Grouch and Snuffleupagus were born," said Palmer. What we now think of as the essence of SesameStreet — the artful blend of fluffy monsters and earnest adults — grew out of a desperate desire to be sticky.

The Distracter, however, for all its strengths, is a fairly crude instrument. It tells you that a child understands what is happening on the screen and as a result is paying attention. But it doesn't tell you what the child understands or, more precisely, it doesn't tell you whether the child is paying attention to what he or she ought to be paying attention to.

Consider the following two Sesame Street segments, both of which are called visual-blending exercises — segments that teach children that reading consists of blending together distinct sounds. In one, "Hug," a female Muppet, approaches the word HUG in the center of the screen. She stands behind the H, sounding it out carefully, then moves to the U, and then the G. She does it again, moving from left to right, pronouncing each letter separately, before putting the sounds together to say "hug." As she does, the Muppet Herry Monster enters and repeats the word as well. The segment ends with the Herry Monster hugging the delighted little-girl Muppet.

In another segment, called "Oscar's Blending," Oscar the Grouch and the Muppet Crummy play a game called "Breakable Words," in which words are assembled and then taken apart. Oscar starts by calling for C, which pops up on the lower left corner of the screen. The letter C, Oscar tells Crummy, is pronounced "cuh" Then the letters at pop up in the lower right-hand corner and Crummy sounds the letters out — "at," The two go back and forth — Oscar saying "cuh and Crummy "at" - each time faster and faster, until the sounds blend together to make cat. As this happens, the letters at the bottom of the screen move together as well to make "cat." The two Muppets repeat "cat" a few times and then the word drops from sight, accompanied by a crashing sound. Then the process begins again with the word bat.

Both of these segments are entertaining. They hold children's attention. On the Distracter, they score brilliantly. But do they actually teach the fundamentals of reading? That's a much harder question. To answer it, the producers of Sesame Street in the mid-1970s called in a group of researchers at Harvard University led by a psychologist named Barbara Flagg who were expert in something called eye movement photography. Eye movement research is based on the idea that the human eye is capable of focusing on only a very small area at one time — what is called a perceptual span. When we read, we are capable of taking in only about one key word and then four characters to the left and fifteen characters to the right at any one time. We jump from one of these chunks to another, pausing — or fixating — on them long enough to make sense of each letter. The reason we can focus clearly on only that much text is that most of the sensors in our eyes — the receptors that process what we see — are clustered in a small region in the very middle of the retina called the fovea. That's why we move our eyes when we read: we can't pick up much information about the shape, or the color, or the structure of words unless we focus our fovea directly on them. Just try, for example, to reread this paragraph by staring straight ahead at the center of the page. It's impossible.

If you can track where someone's fovea is moving and what they are fixating on, in other words, you can tell with extraordinary precision what they are actually looking at and what kind of information they are actually receiving. The people who make television commercials, not surprisingly, are obsessed with eye tracking. If you make a beer commercial with a beautiful model, it would be really important to know whether the average twenty-two-year-old male in your target audience fixates only on the model or eventually moves to your can of beer. Sesame Street went to Harvard in 1975 for the same reason. When kids watched "Oscar's Blending" or "Hug," were they watching and learning about the words, or were they simply watching the Muppets?

The experiment was conducted with twenty-one four and five-year-olds, who were brought to the Harvard School of Education over the course of a week by their parents. One by one they were seated in an antique barber's chair with a padded headrest about three feet away from a 17-inch color television monitor. A Gulf amp;Western infrared Eye View Monitor was set up just off to the left, carefully calibrated to track the fovea movements of each subject. What they found was that "Hug" was a resounding success. Seventy-six percent of all fixations were on the letters. Better still, 83 percent of all preschoolers fixated on the letters in a left-to-right sequence — mimicking, in other words, the actual reading process. "Oscar's Blending," on the other hand, was a disaster. Only 35 percent of total fixations fell on the letters. And exactly zero percent of the preschoolers read the letters from left to right. What was the problem? First, the letter shouldn't have been on the bottom of the screen because, as almost all eye movement research demonstrates, when it comes to television people tend to fixate on the center of the screen. That issue, though, is really secondary to the simple fact that the kids weren't watching the letters because they were watching Oscar. They were watching the model and not the beer can. "I remember 'Oscar's Blending,'" Flagg says. "Oscar was very active. He was really making a fuss in the background, and the word is not close to him at all. He's moving his mouth a lot, moving his hands. He has things in his hands. There is a great deal of distraction. The kids don't focus on the letters at all because Oscar is so interesting." Oscar was sticky. The lesson wasn't.