The Ovine Aviation sketch from the first Monty Python's Flying Circus episode 
 
(A tourist approaches a shepherd.  The sounds of sheep and the outdoors
are heard.)
 
Tourist: Good afternoon.
Shephrd: Eh, 'tis that.
Tourist: You here on holiday?
Shephrd: Nope, I live 'ere.
Tourist: Oh, good for you.  Uh...those ARE sheep aren't they?
Shephrd: Yeh.
Tourist: Hmm, thought they were.  Only, what are they doing up in the trees?
Shephrd: A fair question, and one that in recent weeks 'as been much on my
         mind.  It's my considered opinion that they're nestin'.
Tourist: Nesting?
Shephrd: Aye.
Tourist: Like birds?
Shephrd: Exactly.  It's my belief that these sheep are laborin' under the
         misappre'ension that they're birds.  Observe their be'avior.  Take for
         a start the sheeps' tendency to 'op about the field on their 'ind
         legs.  Now witness their attempts to fly from tree to tree.  Notice
         that they do not so much fly as...plummet.
 
<Baaa baaa...flap flap flap...whoosh...Thud.>
 
Tourist: Yes, but why do they think they're birds?
Shephrd: Another fair question.  One thing is for sure, the sheep is not a
         creature of the air.  They have enormous difficulty in the
         comparatively simple act of perchin'.
<Baaa baaa...flap flap flap...whoosh...thud.>
         Trouble is, sheep are very dim.  Once they get an idea in their
         'eads, there's no shiftin' it.
Tourist: But where did they get the idea?
Shephrd: From Harold.  He's that most dangerous of creatures, a clever sheep.
         'E's realized that a sheep's life consists of standin' around for a
         few months and then bein' eaten.  And that's a depressing prospect for
         an ambitious sheep.
Tourist: Well why don't just remove Harold?
Shephrd: Because of the enormous commercial possibilities if 'e succeeds.