This article is from the of the muppets FAQ, by Craig Silverstein with numerous contributions by others.
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? You know you do.
Seriously, we don't mean to impose our point of view on the FAQ-reading masses. We just love Mahna Mahna, that's all. :-)
(btw, the Mahna Mahna skit is on the video "It's The Muppets: Meet The Muppets," Henson Video VHS 1620.)