Deuteronomy 14:21 says not to cook a young goat in what?

Answer: Its mother's milk

"You must not cook a young goat in its mother's milk" appears in Deuteronomy 14:21 and also in Exodus 23:19.

“You are not to eat any carcass; you may give it to the foreigner residing within your gates, and he may eat it, or you may sell it to a foreigner. For you are a holy people belonging to the LORD your God. You must not cook a young goat in its mother’s milk.”— Deuteronomy 14:21 (BSB)