Peanut butter and jelly sandwich. Get a loaf of bread, a butter knife, a jar of peanut butter and a jar of your favorite jelly. Place them on a table, unopened, and ask the class to instruct you how to make a PBJ. Follow their instructions literally, as a computer would, and much hilarity will ensue. Hopefully, it will help them understand the idea that computer don't "know" anything except what they are told.
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