mikebennidict said:
The boy we couldn't see putting eggs into an egg carton and a cookie. Finally, just before he puts in the 12th egg, he takes the cookie out and eats it. 12 eggs and no cookie.
I actually thought that sounded more like a girl's voice, but...
This sketch was unusual in that (a) a follow-up skit always came immediately after, and (b) said follow-up was one of two different skits, both of which were equally legit.
In one follow-up, Ernie says "Hey that was a good way to count, with those eggs and that cookie." Ernie proceeds to do the same, except with 11 cookies and one egg, since he (and at least a million viewers) likes cookies better than eggs. After he puts them in place, Cookie Monster arrives on the scene and eats the 11 cookies out of the egg carton. Then CM says something like "Oops, me forgot. One egg." CM tossed the egg into the air and caught it in his mouth (how many takes did THAT stunt require?) then trotted off.
In the other follow-up, we see Cookie Monster against a plain background, reacting harshly to the final words of the previous sketch (12 eggs and no cookie). "No cookie? No cookie? I want a cookie!" Then from above an abstract mechanical "puppet" appears and starts talking to CM. His first words were "You're not asking very nicely." The strange puppet tries to convince CM that if he wants a cookie, he should say "Please". At some point the puppet drops a hardcover book, to which CM makes clear his intentions make a meal out of. The puppet then spoke the following line, whose reasoning made no logical sense to me:
"If you say 'please' I'll give you a cookie. If you eat the bookie I'll give you all the cookies you want." IIRC, CM did in fact consume the tome. Then we heard a box open, followed by a shower of cookies to CM's excitement.
Note on that last skit: I was a lot older when I learned what a "bookie" is.