I remember a few skits where Muppet adults didn't believe Big Bird.
When the Count was away, Big Bird agreed to count everything he sees on Sesame Street in his place. When The Count returned, Big Bird handed him a list of everything he counted. The list was, 1 Snuffle-Upagus, 2 Snuffle-Upagus eyes, 4 Snuffle-Upagus feet, 22 Snuffle-Upagus teeth, 1 Snuffle-Upagus tongue, 1 Snuffle-Upagus tail, 1 Snuffle-Upagus snuffle, 4 Snuffle-Upagus feet, 4 Snuffle-Upagus knees....
Then the Count said Big Bird had one wonderful imagination.
And one time, Big Bird's room was a mess and was worried Snuffy would see it. Mumford came along and used his magic to clean the room. Big Bird thanked him and told him Mr. Snuffle-Upagus would love it. Mumford asked, "Snuffle-Upagus? You have some imagination." and he left. Then Snuffy came in and saw the neat room. Big Bird told Snuffy that a magician used his magic to clean the room. Snuffy replied, "A Magician? Aw, Bird. You have some imagination."
And one time, Ernie and Bert were in their apartment talking about Big Bird's imagination. Bert looked out his window and saw Snuffy walking by. He got excited and tried to tell Ernie about it. But Ernie wouldn't come to the window telling Bert that it was just his imagination.