Poem by Christopher Isherwood.

The common cormorant (or shag)
Lays eggs inside a paper bag,
You follow the idea, no doubt?
It's to keep the lightning out.

But what these unobservant birds
Have never thought of, is that herds
Of wandering bears might come with buns
And steal the bags to hold the crumbs.

( Mad as a bag of Frogs IMHO )