Most of you got this right with varying degrees of detail.
Question: why is food that's been cooked in a microwave oven slightly radioactive?
Answer: The microwave has nothing to do with it. It makes the food no more and no less radioactive than it was before cooking. As someone groan-worthily remarked, it's a red herring. Food is naturally slightly radioactive because it contains quantities of naturally-occurring radioisotopes such as 14C.
If you're curious about what levels of radioactivity occur where and why, look here.
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