Chullin 114 - Milk, But Not That of Its Mother
From the phrase, " You shall not cook a young goat in the milk of its mother " we only know that it is prohibited to cook it in the milk of a she-goat. How do we know this for the milk of a cow or an ewe? - We can derive this logically: if the young goat's own mother, with whom he can have offspring, is nevertheless so strict that it is prohibited to cook him in its milk, then a cow, with whom he is not allowed to have offspring (for that would be prohibited interbreeding) - her milk certainly cannot be used to cook him!
However, this logic is flawed. There is a stringency with the young goat and its mother: they cannot be killed on one day. This stringency is not found in regards to the goat kid and a cow who is a complete stranger to him. Thus, with our logic disproved, we need another way to derive the prohibition - and it is derived from the second repetition of " don't cook a young goat in its mother milk ," which, his mother being already prohibited, now teaches us the same law about a cow.
Art: Richard Ansdell - Feeding Goats in the Alhambra