A scientific question

Here's one for all you scientists out there to ponder over.  Well you don't have to be scientists to ponder over it actually, I've spent ages wondering about this one and despite the fact that my A-Levels say that I could be a scientist, I'm really not.


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How is it that when a chicken eats PURPLE cabbage (or as they sell it in the shops Red Cabbage), it poops out GREEN poo?  (For the record, when it eats Green Cabbage, a chicken's poos are perfectly normal) (as are they when they eat Yellow Sweetcorn).

Any thoughts or comments on the matter would be most helpful.  I like to know these things.