The Torah commands that firstborn animals must be consecrated to God. But what happens when the ownership of the animal is complicated? The Mekhilta parses the language of the verse with exacting precision to establish the boundaries of this law.
The phrase "that shall be to you" creates an exclusion. A person who sells his beast to a non-Jew is no longer the owner when the firstborn is delivered. Since the animal is no longer "to you," the firstborn does not carry the sanctity of the firstborn obligation.
But this raises a follow-up question. If selling to a non-Jew removes the obligation, does buying from a non-Jew create it? After all, the animal is now "yours." The Mekhilta anticipates this logic and shuts it down with a second verse: (Deuteronomy 15:19) states "which is born in your herd and in your flock." The emphasis on "your herd" means the mother must have already belonged to you when the firstborn was conceived and born.
Buying a pregnant animal from a non-Jew therefore does not trigger the firstborn consecration. The law applies only when the entire chain — ownership of the mother, conception, and birth — takes place under Jewish ownership. This careful legal reasoning demonstrates how the rabbis used multiple verses to build a complete framework from seemingly simple commandments.