One of the catchphrases of Intelligent Design creationism is ‘irreducible complexity’ – the idea that in some complex biological systems, it’s impossible to remove any one part without causing the whole system to fail. Supposedly this means that such systems could not have evolved but must be the product of a ‘designer’. The term – in its most recent incarnation – was proposed by biochemist Michael Behe, but it’s effectively the same as William Paley’s 19th century concept of the watchmaker.
Behe used to be fond of using the ordinary, bog-standard, everyday mousetrap as an example. I have always found this just a tad unimaginative of him, as while removing (say) the spring would render the mousetrap incapable of doing its current job, this is not the same as saying that the remaining parts do not (& cannot) have some other function. (In a better, biological, example various constituent parts of the so-called ‘irreducibly complex’ flagellum bacteria** do actually have other functions, including adhesion to other cells.) I could, for example, throw the wooden platform of our old mousetrap*** at a mouse. Occasionally I might even hit it.
** Incidentally, there is no such thing as ‘the’ bacterial flagellum.
*** I say ‘old’ because we haven’t used it for a while. These days the fat (6kg) furry ginger monster does the job quite satisfactorily. He probably falls on them.