pesachim 19

a lost needle in a cow draws a pure thread between hands to knives to spit in jerusalem.

pesachim 18

holy cow! turning black to white and white to black. liquids in vessels of all shapes and sizes

anything already pure in contact with the purification waters (water that has the ashes of a red cow mixed in) becomes unpure...

so a cow that has drunk these waters, the waters made with the ashes of a cow, at what point after entering the cow does the purification water become something disgusting.....

(personally I find the whole system of purity/impurity tahara and tamei, and the levels of contagion, quite baffling. not least because a lot of these discussions seem to be taking place after the destruction of the Temple, where the categories of tamei and tahara were relevant. But this case, discussing the status of the water once inside the cow and whether or not contact is defined as just external or also internal contact, seems to highlight the pursuit of the limits of halachic (jewish legal) definitions, even to the absurd and surreal)