Is there any food that is okay if it is eaten raw, but once cooked (after high-temperature cooking), it is poisonous (people will be poisoned if they eat it)?
If this food really exists, then why doesn't it exist? Is it based on some chemical or biological principle?
Jellyfish. 
Jellyfish is a really special food, and all the fresh jellyfish I've eaten are raw, cooked cold or lightly pickled. (It is completely fresh, not salted, salvaged ashore and died naturally, and then eaten and mixed on the spot).
Because the jellyfish itself contains more than 90% water, and the structure is fragile and single, this kind of red sting is basically salvaged for more than ten minutes, and the stinging cells and accompanying neurotoxins in the body are completely ineffective. At this time, the jellyfish, basically as long as it is cut and washed, you can add a variety of side dishes and seasonings to mix cold, the jellyfish skin tastes smooth and tender, like cold powder; The jellyfish head is crisp and springy, and the seafood is rich.
But if you boil it in water, it will melt directly; Stir-frying in oil is even more outrageous, and collapse is everywhere; If steamed, the jellyfish will disappear in place on the plate after boiling; Frying, then it is not fried jellyfish but fried kitchen.
On the contrary, fresh jellyfish have never seen cooked food. (Dehydrating fresh jellyfish with alum or salting to make a water jellyfish and then cooking it should not be counted, because dehydration and pickling itself is also a way of aging.) ）
Flaxseed oil, as well as oils that are not heat-tolerant such as walnut oil.
These oils and fats mainly contain polyunsaturated fatty acids, which are good for health at low temperatures, but when heated, they are easy to react with oxygen in the air, resulting in oxidation of fatty acids, that is, thermal deterioration. Therefore, these fats are generally only used for cold consumption and should not be heated.
Ice grass, this thing can only be made cold mix, I haven't heard that it can be fried, the vegetable market will always find intentionally or unintentionally, ice grass has been used on the table for a long time, from the gold when it was just introduced, and now it is to the people's homes, but it is not good for this mouthful, I once wanted to make a hot dish, with our decades of cooking experience, and I have not yet caught it, but the pot is turned over, the ice grass will not only melt after heating, but also produce a rotten smell, I never waste, I can only look at the dish and sigh.