Tevul Yom, Chapter 3, Mishnah 3. This chapter is occupied with the question of chibur, connection: when a tevul yom touches one point of a food, how far does his touch reach? Since a tevul yom invalidates terumah that he touches, everything turns on how much of the dish counts as joined to the spot under his finger. Our mishnah brings the question into a kitchen, through a dish that was familiar to everyone in the days of the mishnah.
The mishnah opens: "Yarak shel terumah u'beitzah terufah netunah al gabav" - vegetables of terumah, with a beaten egg poured over them. This was a common way of preparing vegetables: an egg was whipped and spread across the top of the vegetables as a layer that gave the dish its flavor. The pan was then baked in the oven until that egg set and congealed into a solid sheet resting over the vegetables beneath it.
Now to the case itself: "naga tevul yom ba'beitzah" - a tevul yom placed his finger on the baked sheet of egg. The Tanna Kamma rules "lo pasal ela kelach sheknegdo": the only thing rendered unfit is the one stalk lying directly under the spot where his finger came to rest, and nothing beyond it. All the rest of the vegetables in the pan remain tehorim.
Rabbi Yosi takes a stricter position. He rules "kol haseder ha'elyon": every vegetable in the topmost layer is disqualified. As he sees it, the hardened egg joins together all the vegetables it is pressed against, so that contact made at any one place on that sheet travels along it and reaches each of them.
The mishnah closes with a case that changes the picture entirely: "im hayah k'min kova" - if the egg was shaped like a helmet. In the course of baking, the egg would sometimes puff and dome upward, so that it arched over the vegetables without actually resting on them. If a tevul yom touched an egg in that state, "eino chibur": there is no connection at all between the egg and the vegetables, and the vegetables remain tehoros.
And what of the egg? The egg plays no part in the question of what became invalid, since eggs simply do not fall under the laws of terumah; an egg has the status of chulin. And chulin suffers nothing from the contact of a tevul yom. So the whole discussion here revolves around the terumah vegetables alone, and they are endangered only when the egg he handled is truly pressed against them.