that "Am I being detained?" is the phrasing we have to use with cops because we're trying to limit the number of phrases we use so they can't be taken out of context.
"I'm not answering questions" is so by far a politer way of declining to answer a question that any other response would seem extra rude in normal conversation. The cops know why we're saying "am I being detained?" when they ask a question because they know full well they are asking us to waive rights by asking a question in the first place. It's more fair to hold them accountable to their reaction to it than just some random person being paid to run kids out of a parking lot.
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