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In brief, the “popcorn” button was initially introduced by fancy high-end microwaves that used an integrated humidity sensor to tell when your popcorn was done; microwaveable popcorn vents steam as it cooks, so by monitoring the amount of steam in the cooking chamber, you can get pretty close to perfectly popped popcorn every time (though it’s generally only pretty close, since different brands of microwaveable popcorn have different moisture content).

As the feature became popular, manufacturers of cheap microwaves started adding a button labelled “popcorn” as well, in order to imply that they offer this feature. These “popcorn” buttons simply run the microwave for a fixed amount of time that the manufacturer figures is close enough to the printed cooking time of most commercial brands.

In practice, of course, the fixed-time “popcorn” button usually just sets your popcorn on fire. To make matters worse, owing to America’s permissive advertising laws, microwave manufacturers are allowed to make all sorts of misleading-but-technically-true statements in their packaging and instruction manuals, rendering it nearly impossible to tell whether a given model of microwave has a real humidity-sensing “popcorn” button or a fake fixed-time “popcorn” button before buying it.

In summary: the “popcorn” button that your microwave popcorn instructs you not to use exists because American microwave manufacturers are using a misleadingly labelled button in order to imply that their product has a feature that it does not in fact have, in a way that can potentially trick people into burning their houses down, for advertising purposes. This is perfectly legal.