Nintendo puts a "bittering agent" on game cards so people don't stick them in their mouths

Kotaku recently contacted Nintendo to ask them why Nintendo Switch cartridges taste so awful. Nintendo replied: "To avoid the possibility of accidental ingestion, keep the game card away from young children. A bittering agent (Denatonium Benzoate) has also been applied to the game card. This bittering agent is non-toxic."

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According to Wikipedia, denatonium benzoate is the most bitter chemical compound known, commonly used as an aversion agent to prevent accidental ingestion, which is why the Switch cards are coated in it. It's also used in animal repellent, shampoos, soaps and nail-biting prevention.