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Dear Dr. Staff: Why does popcorn pop in a microwave oven? I read in a science magazine that things called quasars in outer space emit microwaves, but I held a bag of popcorn up to the sky one night for a whole hour, and none of it popped. Why do microwaves make popcorn pop only in an oven?

--Perplexed Popcorn Popper

Dear Popper: You've absolutely right that both microwave ovens and quasars produce microwaves. But the two things produce very different microwaves.

In order to answer to your question, one should consider exactly what microwaves are. Everyone knows what waves are, and every wave has a particular size, called its wavelength. ``Micro'' means ``small''; microwaves are waves with small wavelengths. Corn kernels are small things, too, which is why they get popped by small waves. However, each kernel has its own unique size, which corresponds to the wavelength that can pop it. Microwave ovens work by sensing the size of the popcorn kernels you've placed in them (that's why the kernels have to sit in the oven for a couple of minutes before they start to pop--the oven is sensing the kernels). The oven then transmits a microwave to pop each kernel.

But quasars don't know how big your popcorn is--they just spit out microwaves in all sizes. Most of these microwaves aren't the right size to pop your corn. If you had held the corn up to the quasar for long enough, a microwave of just the right length would probably happen across one of your popcorn kernels, and pop it; it just doesn't happen very often.

You may have seen old popcorn poppers in antique stores that used hot oil or hot air to pop corn--people used to pop corn in these devices before they had microwave ovens. They worked by heating oil and air until they produced lots of microwaves in roughly the right frequencies to pop the corn. But they were inefficient, because they didn't have the sensors that microwave ovens do, and they wasted energy because, like the quasar, their wavelengths of the microwaves weren't tuned to the corn--lots of waves didn't pop the corn, but instead just heated the air and oil. Everyone has abandoned those old devices, in exchange for modern, efficient microwave ovens.

--Major Microwave Maven

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