David Mamet's guide to movie gags--

"All right, what about magnets? Cat, with magnet hidden (or, indeed, implanted) in its neck, is lured to stick its head into a funnel smeared with tuna fish. Funnel contains radio-controlled electromagnet. Two prop guys hold monofilament lines, one to ootz cat and funnel over the ledge, second to ootz it back. Electromagnet is turned off, and grateful cat removes head from funnel. In this scenario, I would shoot the cat sequence first. That is, before "establishing" the cat.

Or: set up four or five ledges, four or five different cats. Whichever cat-and-team first got the shot in the can, that cat would be the Hero Cat, and play in the rest of the film. This version, of course, would require many, many cats standing by. "