Eyes have long been an important part of the Muppet aesthetic. For example, one of the most distinctive aspects of Kermit the Frog is his eyes, originally made from ping-pong ball halves and with unique cross-lined pupils.
The Magic Triangle
The placement of the eyes on a Muppet character is key to the success of the character. In The Art of the Muppets, the Henson Associates staff wrote, "Perhaps the single most important aspect of the Muppet look is the set of the eyes in relation to the nose and mouth. The Muppet people call this the 'magic triangle': correctly positioned, it creates a central focal point essential to bringing a puppet to life in the eye of the camera -- and therefore the viewer." 
The development of this concept is often credited to Don Sahlin, chief architect responsible in many ways for the basic look of the Muppets.  Jim Henson explained the importance of eye placement: "It would be the last thing [Sahlin] would do, and he always wanted me there, to make sure it was right for both of us -- making sure the eyes had a point of focus, because without that you had no character." 
The focus of a Muppet's eyes depends on the placement of the pupils. The pupils are rarely in the exact center of the eye; instead, they are placed toward each other, making the character slightly "cross-eyed." This creates the illusion of focus, and makes it easier for the puppeteer to emphasize what the character is looking at. 
Because the eyes are such an essential element to the mechanics of the characters, Muppet builders usually wait until the last minute to add the eyes. 
Eyes and Age
The eyes of Muppet characters have often been broad and inviting, in the manner of cartoon character's eyes. Much in the manner of newspaper cartoonists, the size of the character's pupils have been a useful means of telegraphing a character's age. Sesame Workshop has explained this concept in a 2006 newsletter: "Muppet designers use different sized pupils depending upon how young or old they want a Muppet to look. The smaller the pupil, the older the Muppet looks; the larger the pupil, the younger the Muppet looks." 
When the Muppet designers created the "Land of Gorch" characters for Saturday Night Live, Henson insisted on giving them taxidermy eyes -- realistic-looking glass eyes used for stuffing animals. The eyes lent a more naturalistic look to the characters, contrasting with their general abstract nature. 
Later productions used taxidermy eyes to blend the look of a real animal with comic anthropomorphism, as seen in such characters as Eliot Shag or Jake the Polar Bear. Creature Shop productions The Dark Crystal, Labyrinth, and Dinosaurs used taxidermy eyes almost exclusively.
For the majority of Muppet characters, the eyes are fixed in certain positions and essentially static. Since Whatnots and Anything Muppets have their features re-arranged frequently, the eyes are simple attachments. Once tacked on, the eyes remain there unless forcibly removed by another character. These eyes have no moving parts. Thus any eye contact or movement is generally an illusion created by the puppeteers through performance, or through calculated manipulation and clenching of the puppet's face at key moments. The most recent example of this clenching method is evident in Elmo -- performer Kevin Clash is able to make the character look up in pondering or appear to raise an eyebrow by slightly adjusting the structure of the puppet's head where its eyes are attached.
For other characters, a variety of techniques have been used, both sophisticated and simple. Cookie Monster, for example, has googly eyes, created by pinning the pupils loosely onto the eyeballs, which gives him an excitable look and makes the character appear more animated.
Other creations, such as Big Bird, Animal, Mr. Snuffleupagus, Hoots the Owl, Boo Monster and Telly Monster have complex eyes with lid mechanisms, which can open, close, expand or contract to create different expressions. Mahna Mahna and Floyd Pepper have had blinking eyes, unique in their design in that they are constructed simply as hollow "sockets," while in his debut in The Frog Prince, Sweetums' eyes lit up. One of the more advanced techniques used on a simple Muppet character is in the eyes of Wembley Fraggle. His spherical eyeballs are rigged with a mechanism that allow his pupils to appear to move within the realm of the eyes' white space. Unlike Sam the Eagle's pupils, which only move left to right, Wembley's can move in any direction, and often in a rotating manner to create the illusion that he is "rolling his eyes."
Still other characters are expressive by means of the material around their eyes. Bert's eyes have always been fixed, but his brow is extremely mobile. Dr. Teeth wears a pair of sunglasses which consist only of an upper half, suggesting eyelids at rest for a laid-back appearance, but which can be flung back at a moment's notice to telegraph shock or excitement.
In some cases when a scene requires total darkness (such as the electricity going out), this is done by using standalone eyes resembling the character's eyes on a stick against a black background, a variation of the typical cartoon convention of eyes floating against the darkness. This can be seen in a 1969 sketch when Ernie and Bert blow a fuse running too many electrical things at once and cause a blackout, and in a later segment when Kermit attempts to demonstrate "light" and "dark" while Grover crashes around blindly in the darkness.
Glasses and Obscured Eyes
Eyeglasses, monocles, sunglasses, and other spectacles are often useful not just as accessories, but as a defining part of a character's features. The eyes of Scooter and Herbert Birdsfoot are permanently attached to their glasses, while Pops' spectacles hide a pair of perpetually squinting peepers. Bunsen Honeydew and Zoot have no visible eyes at all, with their glasses essentially functioning as eyes.
Still other characters have no visible eyes whatsoever. Wendell, Mel and Boober Fraggle are key examples, with the latter's "eyes" implicitly obscured by his mop of hair and cap, in the manner of Beetle Bailey.
Lashes, Lids, and Brows
Simplicity and suggestion can be just as effective as the most complex eye mechanism in shaping a character's personality. Miss Piggy's carefully crafted purple lids and lashes, placed over blue pupils, suggest strength and glamor. Waldorf's age and affinity for napping are suggested by the deep set of his eyes. Janice's eyes are simply a pair of angled lashes, while the Amazing Mumford and The Swedish Chef possesses nothing but a pair of bushy eyebrows.
Eyebrows often emphasize (or exaggerate) certain personality traits; aggressive Muppet monsters usually have large black eyebrows, whereas the mild-mannered Kermit doesn't have any. 
There are cases in which eyelids are built for a character who does not normally have them, or when the eyelids that they do have are modified to cover their eyeballs. This is done for scenes in which a character is required to sleep on screen, when the simpler solution of turning the eyes away from the camera will not work.
Likewise, a slight off-camera tilt of a character's eyes between shots will do wonders for conveying emotions. A drastic, unexpected exchange of a set of eyes with another one, as seen with the Happiness Hotel gang in The Great Muppet Caper and Captain Abraham Smollett in Muppet Treasure Island, can induce hilarity.
- Sesame Street uses approximately 219 pairs of eyes (and 180 noses) per season.
- ↑ The Art of the Muppets, p. 7. New York: Bantam Books/Muppet Press, 1980.
- ↑ Christopher Finch, Jim Henson: The Works, p. 65. New York: Random House, 1993.
- ↑ Inches, Allison. Jim Henson's Designs and Doodles, p. 50. New York: Harry N. Abrams, Inc., 2001.
- ↑ Cheryl Henson and the Muppet Workshop, The Muppets Make Puppets!, p. 19. New York: Workman Publishing, 1994.
- ↑ Cheryl Henson and the Muppet Workshop, The Muppets Make Puppets!, p. 18. New York: Workman Publishing, 1994.
- ↑ Weekly Trivia, Sesame Family Newsletter, February 22, 2006
- ↑ Christopher Finch, Jim Henson: The Works, p. 86. New York: Random House, 1993.
- ↑ Cheryl Henson and the Muppet Workshop, The Muppets Make Puppets!, p. 25. New York: Workman Publishing, 1994.
- ↑ Sesame Family Newsletter, July 30, 2008