Your question doesn't make sense

"Your question doesn't make sense" (also known as obsfucating confusion question) is a question purposefully asked out of context to prevent an individual figuring it's true purpose or evading it.

Context
Questions often asked in context can often alert suspicious individuals to whether or not they are likely to get found out, and thus likely either scare them into going quiet, evading the question, changing their story, or lying.

By asking the question out of context, it prevents the individual from figuring why it's been asked (and doesn't give them grounds to not answer it). Non-sequitur questions are common within social situations, usually as last-minute questions the individual was only just reminded of to ask "Oh, I nearly forgot to ask, is Quack the duck red or blue?".

By asking out of context, the individual is often unprepared, has a different set of memories loaded (which means they must load the memories related to the system, and as the mind is particularly efficient, it tends not to load unnecessary memories along with it, IE, the lies, which means it's more likely to be truthful), and doesn't give them enough time to establish a well-constructed lie (by ignoring the question they seem rude, and by avoiding it, they seem paranoid so it doesn't give them much time to react).

A common response is "your question doesn't make sense", in which their subconscious is protesting the fact you've asked the question out of context and caused them such hassle. It might not be in context, but it is convenient to ask when they're around.

Usage
Individuals should use spacing and avoid the urge to follow up the questions with additional ones (which is natural curiousity), as the chain of questions would likely give away the line of reasoning and thus that you were close to exposing a lie. Preferrably one question per topic, with as much spacing between questions as possible. One may alternately ask other curious questions that are unrelated to your chain of inquiry, so for example "Oh, I forgot to ask, is Quack the duck red or blue? And also, do you like animals by any chance?".