From the Muppet FAQ:

What’s the best ever Muppet sketch?

Mahna Mahna, with Mahna Mahna and the two Snowths. Even now, you sing « doo doo do doo doo » under your breath every time someone says « phenomena, » don’t you?

And just what is a Mahna Mahna?

[via Fizzz and Memepool]