Aside from the prohibition against cooking meat and milk together, there is also a prohibition against eating them once they were cooked or deriving any benefit from the mixture whatsoever. Several sources are offered in the Gemara for these three related rules, the most famous of which appears on today’s daf (=page) –
The school of Rabbi Yishma’el taught: “Thou shalt not seethe a kid in its mother’s milk,” is stated three times (Shemot23:19, 34:26, 14:21); one is a prohibition against eating it, one a prohibition against deriving benefit from it, and one a prohibition against cooking it.
Another approach is offered by Rav Ashi, who says that we learn that it is forbidden to eat meat cooked with milk from the passage in Sefer (14:3) that prohibits eating “any abominable thing.” Rav Ashi interprets that to mean that anything considered “abominable” by the Torah – i.e. anything produced from an activity that the Torah prohibits – may not be eaten.
Based on this logic the Gemara wonders why other prohibited activities do not forbid what they produce. The Gemara even offers a mnemonic list of such actions:
‘Shabbat‘ – Rashi explains that this refers to Shabbat activities such as cooking on Shabbat. Generally speaking, we distinguish between food cooked on Shabbat by accident and if the transgression was done purposefully. While there are different opinions on the resulting prohibitions, according to the majority of the Sages, even if the food was cooked on purpose it will be permitted to others after Shabbat is over.
‘Plowing’ refers to using an ox and a donkey together while plowing a field, a biblical prohibition ( 22:10), understood by the Sages as prohibiting the use of any two animals that are different for such purposes.
‘Diverse kinds of seeds’ refers to what the Torah calls kilayim, i.e., planting different seeds together. Only seeds planted in a vineyard will create a situation where the produce is forbidden; all other types of kilayim may not be planted, but can be used after-the-fact.
‘It and its young’ refers to the Jewish law prohibiting the slaughter of a mother animal and its offspring on the same day (see above, daf 78).
‘Letting the mother bird go from the nest’ refers to the law forbidding someone from taking the eggs or hatchlings from a nest without first chasing away the mother bird (see 22:6-7).
In contrast with milk and meat, in each one of these cases the Gemara maintains that there is scriptural evidence that the prohibited activity will not forbid the object that is produced.