The Torah describes the Israelites in Egypt dipping hyssop into blood "which is in the saf." The Mekhilta records Rabbi Yishmael's reading of this enigmatic word, and his interpretation transforms the scene from a simple bowl of blood into something far more vivid.
The word saf can mean either "basin" or "threshold." Rabbi Yishmael insists it means threshold. The verse is telling us that each Israelite household made an indenture — a hollow depression — at the side of their doorway's threshold and slaughtered the Paschal lamb so that its blood collected directly in that carved-out space in the ground.
This was not blood carefully collected in a kitchen vessel and carried to the door. This was blood flowing at the threshold itself, pooling in the stone or earth of the doorway. The hyssop was then dipped into this threshold-pool and applied to the lintel and doorposts above.
Rabbi Yishmael supports his reading with two verses from the prophets. (Ezekiel 43:8) uses the related word sippam — "their threshold" — in the phrase "in setting their threshold by My threshold," clearly referring to a physical doorway structure. And (Isaiah 6:4) describes "the posts of the sippim" shaking, again using the word in the sense of a threshold or doorframe.
The combined evidence makes Rabbi Yishmael's case: the blood of the Paschal lamb was not in a bowl. It was in the ground at the entrance of every Israelite home — a visible, unavoidable mark of covenant at the very place where the Angel of Death would pass.