Not a teacher
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Here, the figure of authority, Rover_KE, has masterfully demonstrated how to use magical invocations—in this case, a simple please rather than a more advanced abracadabra—to effectively invoke a sense of politeness into the addressee's mind, even though this imperative sentence is not asking a favor but a command, which only proves that a magic word could be used to make an imperative more polite. Remember, however, that this practice should be reserved only for the most distinguished weavers of the black arts.
A simple muggle might want to use something simpler, like "Could you bring me some ketchup?" or "Can I have some ketchup?", or even a slightly over-the-top "Excuse me, good sir, but it has come to my attention that you seem to have forgotten to bring me the sachet of ketchup that was meant to be part of my order, which I have already paid for and was expecting to get without the need to ask for. Would you kindly rectify that mistake?"
I'll be crawling back to my hovel now...