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Spaghetti
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That's because there isn't really such a thing as "microwave safe plastic"
You don't even need to heat the plastic for the sauce to stain it. Fats want to bond with plastics, making plastics harder to clean + tomato sauce is awesome at staining. Heating might make it worse, but it's not necessary.
Yep. If it stains that means it's porous and is likely leeching plastics or plasticizers.