If you try and take a cat apart to see how it works, the first thing you have on your hands is a nonworking cat.

Douglas Adams (1952–2001) English author, humorist, screenwriter
Speech (1998-09), “Is There an Artificial God?”, Digital Biota 2 conference, Cambridge, UK
    (Source)

Discussing the historical mysteriousness of "life" and why it was (and is) difficult to understand.

Collected in The Salmon of Doubt, Part 2 "The Universe" (2002) [ed. Peter Guzzardi]. This (and other) sections of this extemporaneous speech were also quoted in Richard Dawkins, "Eulogy for Douglas Adams," Church of Saint Martin in the Fields, London (2001-09-17).

Variant: "If you try and take a cat apart to see how it works, you end up with a non-working cat. Do not try this."