"Seven days shall it be with its mother" — the Torah requires a first-born animal to remain with its mother for seven days before it can be given to a Kohen (a priest). But the Mekhilta asks: what does "with its mother" mean? Does the animal need its mother to be alive, or can it fulfill the seven days even if its mother has died?
(Leviticus 22:27) uses slightly different language: "It shall stay seven days under its mother." The word "under" could be read literally — meaning "in place of" or "instead of" its mother. This would imply the seven-day period applies even when the mother is dead.
The Mekhilta rejects this reading by cross-referencing the two passages. Here in Exodus, the Torah says "with its mother" — clearly meaning the mother must be alive and present. The Leviticus verse must be read in light of the Exodus verse: "just as here, with its mother, so there, with its mother." The "under" in Leviticus does not mean "instead of" but "alongside" — the mother must be alive.
The practical consequence is that a first-born animal whose mother dies within the first seven days cannot complete the required waiting period. The mother's survival is a condition of the seven-day rule. This cross-referencing technique — using one verse to control the interpretation of another — is a hallmark of the Mekhilta's method, ensuring that related passages are always read as a coherent whole.