who knows if the moon's a balloon, coming out of a keen city in the skyöfilled with pretty people?

-cummings, e e pen name of  Edward Estlin Cummings
  'Seven Poems, VII'. David Niven used the phrase for his autobiography, The Moon's a Balloon (1975).

If I laugh on that particular day I become so filled with Laughing Gas that I simply can't keep on the ground. Even if I smile it happens.The first funny thought, and I'm up like a balloon. And until I can thinkof something serious I can't get down again.

-Travers, P(amela) L(yndon)
  MrWigg, Mary Poppins's uncle.The'particular day' is when his birthday falls on a Friday. Mary Poppins, ch.3.