Uktzin, Chapter 3, Mishnah 4. This chapter examines what qualifies as an ochel, a food, for the purposes of tumah and other halachos. Sometimes an item needs machshava, a person's designation, to enter that category. Our Mishnah looks at the opposite movement: an item that was genuinely a food and then lost that status altogether.
The Mishnah teaches: "Hasheves, mishenasan ta'am bakdeirah, ein bo mishum terumah, ve'eino mitamei tumas ochlin." Sheves, which many explain as dill, once it has given its flavor into the pot, is no longer subject to the halachos of terumah, and it does not become tamei with the tumah of foods.
The logic turns on the role dill plays. Nobody wants dill for its own bulk; the whole point of adding it is the taste it contributes. Once it has been simmered together with a soup or any other dish a person is preparing, its taste has migrated into the food around it. What is left behind is exhausted: the quality that gave it any value has departed, and the leaves floating in the pot carry no taste whatsoever.
Two consequences follow. First, if this was dill of terumah, a zar, a non-kohen, may now eat it. Terumah applies to food, and this dill no longer carries the halachic status of food once it has surrendered its taste. Second, it is no longer susceptible to tumas ochlin. The laws of food tumah attach themselves to an ochel, and an item emptied of the very quality that made it useful has stepped out of that category.
The Mishnah thus teaches a broad and important lesson: food status is not a permanent label stamped on a substance. It tracks the item's actual usefulness to a person. So long as the dill held its flavor, it was terumah and it could contract tumah. Once the flavor passed into the pot, the dill became, halachically speaking, nothing more than spent leaves.