Have you ever taken a lie detector test?

I was watching a show and this guy had to take a lie detector test and this came to mind.

A secondary question, though, is if you’re trying to cheat why can’t you ask yourself your own question, in your mind, and answer that question?

For example, “Did you kill Bill?”
Let’s say you did, so you want to lie. Why can’t you ask yourself a question like, “Have you ever robbed a bank,” and, assuming you haven’t, answer that question out loud?