Moses told the people, "Be ready in three days" (Exodus 19:15), instructing them to separate from their wives in preparation for receiving the Torah. But the Mekhilta notices a problem: when God originally gave Moses the instructions in verse 11, He said "have them ready for the third day" without explicitly mentioning separation from wives. So where did Moses get the additional requirement?
The answer comes through a rabbinic interpretive technique called gezeirah shavah — a verbal analogy in which identical words appearing in two different passages are used to transfer legal meaning from one context to the other. The phrase "be ready" appears both in God's instruction (verse 11) and in Moses's instruction to the people (verse 15). In Moses's version, "be ready" is explicitly linked to marital separation. The gezeirah shavah allows the rabbis to read that same meaning back into God's original command.
Just as "be ready" in verse 15 clearly implies separation, so "be ready" in verse 11 also implies separation — even though the text does not spell it out. The identical language creates a bridge between the two verses, carrying the specific meaning of one into the other.
This passage illustrates one of the most powerful tools in rabbinic interpretation. The Torah is understood to be perfectly precise in its word choices. When the same phrase appears in two contexts, it is never coincidental. The repetition is an invitation to connect the passages and transfer meaning between them. Moses did not add to God's instructions. He unpacked what was already embedded in the word "ready."