Zavim, Chapter 4, Mishnah 1. Although the tractate is named for the zav, its halachos apply equally to a nidah, who likewise conveys tumah by sitting, lying and leaning, and who renders objects tamei with tumas midras. This mishnah explores cases where the nidah never touches the object at all, and yet her weight, transmitted through something that shifts or sags beneath her, is enough to make that object tamei as though she had sat upon it directly.
The mishnah opens: "Rabbi Yehoshua omer: nidah sheyashvah im hatehorah bamitah, kipah sheberoshah temei'ah midras", Rabbi Yehoshua says that if a nidah sat together with a tahor woman on a bed, the head covering worn by the tahor woman becomes tamei with tumas midras. The bed here is not anchored to the floor. When the nidah settles onto it, her weight causes the bed to give and dip, and that shift travels through to the other woman. The result is that even the cap on her head is treated as though the nidah had actually sat upon it.
Rabbi Yehoshua applies the identical reasoning to a boat: "yashvah basfinah, kelim sheberosh hanes shebasfinah temei'in midras", should she sit down in a boat, the utensils resting up at the head of the mast contract tumas midras. The case is a light craft, the sort that dips and tilts under a passenger's weight. One observation governs both halves of the ruling: a bed gives way and a boat rides down into the water, and once the whole structure sinks even slightly beneath the nidah, her weight is regarded as pressing upon everything that structure holds, including the utensils perched at the very head of the mast.
The mishnah continues with a case where the nidah is carrying rather than sitting: "noteles arevah mele'ah begadim, bizman shemasa'an kaved, temei'in", if she lifts a trough filled with garments, then when their weight is heavy, the garments are tamei with tumas midras. The rationale is the same idea of weight-bearing seen from the other direction: a heavy load pulls her down, so that she is effectively leaning upon it and hanging her weight on it, and that leaning conveys tumas midras to the garments. "Bizman shemasa'an kal, tehorin", when their weight is light, they remain tahor, because a light load does not draw her weight onto it, and there is no leaning to speak of.
The closing clause marks the boundary from the opposite direction: "zav shehikish al hakitzutzah venafal kikar shel terumah, tahor", a zav banged on a balcony, and a loaf of terumah that had been lying on it fell down, yet the loaf is not rendered tamei. What accounts for this? The balcony is a solid, firmly built structure. His blow did not budge or unsettle it in the slightest, and therefore the falling of the loaf cannot be charged to his heseyt. What brought the loaf down was a tremor, a shudder that ran through the structure and shook the bread loose, and such a quiver is neither leaning nor pressing of weight, so it transmits no tumah. The loaf of terumah retains its taharah.
Taken together, the mishnah teaches us to look at where a person's weight actually comes to rest. A swaying bed, a rocking boat and a heavy load all carry the weight of the nidah onward to whatever they hold, and the tumah of midras travels with it. A firm balcony carries nothing of the zav's weight, and a mere shudder passing through it leaves the terumah upon it tahor.