Microwaving metal always causes fires and explosions.
While metal in a microwave can cause sparks and fire under certain conditions, smooth metal objects (like a spoon in a cup) are often safe. The danger depends on shape, edges, and the presence of arcing paths.
What changed?
The rule spread the way cautionary rules do: through warning labels, through parents, through the accumulated authority of "everyone knows." Put metal in a microwave and it will spark. Spark means fire. Fire means disaster. By the time microwave ovens became standard kitchen appliances in the late 1970s and 1980s, the prohibition on metal had hardened into something that felt like physics, an absolute, categorical rule that brooked no exceptions.
The actual physics is considerably more nuanced. What causes problems in a microwave is not the presence of metal but specific geometric conditions. Thin metal, aluminum foil with crinkled edges, a twist tie, a fork with widely spaced tines, can concentrate the microwave's electric field at sharp points and edges, creating a potential for sparking or arcing. A smooth, rounded metal object, a spoon resting in a mug, a small stainless steel container with no sharp protrusions, often passes through a microwave cycle without incident. The shape matters, not the category. Microwave engineers have known this since Percy Spencer accidentally discovered the microwave effect in 1945 and the first commercial ovens followed.
Some of this nuance appeared in the original appliance manuals, which cautioned against metal with sharp edges and thin foils rather than banning metal categorically. But consumer education simplified the instruction to its most cautious form, which is how safety rules tend to travel: the exception gets dropped, the worst case becomes the universal rule. Generations grew up believing that a stainless steel spoon forgotten in a soup bowl would trigger an explosion.
The simpler rule is not entirely wrong as a practical heuristic, glass or ceramic avoids the question entirely, and no one is harmed by extra caution in the kitchen. But the "always" was never true, and believing it means treating a safety guideline as a law of nature. The microwave oven, invented by accident when a radar engineer noticed a chocolate bar in his pocket had melted, was never as dangerous as its reputation suggested, and never as safe as the absence of warnings would imply. The truth, as usual, required reading the footnote.