Rabbi Nathan expanded the scope of the deposit laws beyond their most obvious application. The Torah says that when someone deposits "money" with a neighbor for safekeeping, certain obligations arise. Rabbi Nathan taught that "money" includes money of second-tithe — funds designated for the purchase of food to be eaten in Jerusalem.
This is significant because second-tithe money has a special status. Unlike first-tithe, which goes to the Levites, second-tithe reverts to the owner for personal use within Jerusalem. It is the owner's money with a sacred designation. When such money is deposited with a neighbor, the deposit laws apply to it just as they would to ordinary funds.
The Torah also mentions "vessels" — and Rabbi Nathan taught this includes any vessel, not just valuable or specific ones. Any physical object entrusted to another person falls under the deposit laws.
But the Mekhilta pressed further. We now know about money and vessels. What about other types of property — livestock, produce, or anything else? The Torah answers with the phrase "to watch." This inclusive language extends the deposit laws to anything that requires watching. Money, vessels, animals, crops — if it was given to someone to guard, the guardian's obligations apply.
The Mekhilta systematically expanded a seemingly narrow verse into a comprehensive law of bailment covering every category of property that one person might entrust to another.