Chullin 108 - A Drop of Milk
If a drop of milk fell onto a slice of meat that was cooking in a pot, then if the drop contained enough milk to impart a flavor to that slice, it is prohibited. Since in this case no one was stirring the pot, the milk spread throughout the piece on which it fell but no further.
If someone was stirring the pot when the drop fell in, the milk was not absorbed into that slice on which it fell, but continued to travel through the slice out into the soup, and dispersed throughout the entire contents of the pot. Now if the drop contains enough milk to impart a flavor into that entire pot, it is prohibited.
Rav said, "In the ruling's first case, the words 'it is prohibited' refer not only to that piece, but to the entire pot." Why?! According to Rav, the complete slice, having become forbidden because of a drop of milk, acquires the status of prohibited meat, or nevelah. Now there is much more prohibited matter in the pot, affecting the whole. Furthermore, Rav follows Rabbi Yehudah, who says that like matters (meat in meat) never become nullified.
Art: Jean Baptiste-Simeon Chardin - The Kitchen Table