This is one of the many Pet Shop sketches. Cleese is the customer and Palin is the shopkee

This is one of the many Pet Shop sketches.  Cleese is the customer and Palin
is the shopkeeper:

C: Good morning, I'd like to buy a cat.
P: I've got a lovely terrier.
C: No no I want a cat really.
P: Ah how 'bout this then?
C: Well that's a terrier.
P: Well it's as near as dammit!
C: What do you mean, I want a cat.
P: Look, I'll tell you what - I'll file his legs down a bit, take his snout out,
   stick a few wires through his cheeks, make a lovely cat that would.
C: No no no no no, it's still not a proper cat.
P: What do you mean?
C: Well it wouldn't meow.
P: Well it would howl a bit.
C: No no no. Um, have you got a budgie?
P: Ah no, fresh out of them. I'll tell you what, I'll lop its back legs off,
   make good, strip the fur, stitch on a couple of wings, and staple on a beak
   of your own choice. No problem, lovely budgie!
C: Well how long would that take?
P: Ooh conversion like that, ah back legs off, stripping, ooh ah Harry, could
   you do a budgie job on a terrier straightaway?
Harry (in back room): I'm still putting the tuck in the airedale, 
   then I got the frogs to let out!
P: Friday?
C: No no, I need it tomorrow. It's a present.
P: Oh long job a budgie. Tell you what, a terrier makes a lovely fish. I could
   do that for you now. Legs off, fins on, simple metal tube through the back
   of his head so it could breath, bits of gold paint, make good?

: You'd need a big tank!
P: It's a great conversation piece!
C: Yes, yes, allright, but um ... only if I can watch!