Why do some foods make you gag?

There is pretty much no way a nasty piece of food can get past my stomach.

Mouldy toast, off yogurt, under-cooked egg white, a little brown eye bit from a potato, even just watching a student member of the rugby team eat a packet of butter ; all these foods (and probably many more, but my mind has gone blank) have caused me to gag, and often barf until it hurts. Just writing about these is making my gag reflex get all pumped and ready.

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