Machshirin, Chapter 5, Mishnah 2. This chapter continues to work out when water that comes onto a person, or onto produce, counts as water that arrived with a person's consent. Only such water is bechi yutan, water that can serve as hechsher and render food susceptible to receiving tumah. Water that arrives against a person's will, or with no interest of his at all, leaves the food exactly as it was.
Swimming, and the Water That Flies Off
The Mishnah begins: "Hashat al pnei hamayim, hanitazin einan bechi yutan" - regarding someone floating along the surface of a body of water, the droplets scattered by his motion do not have the status of bechi yutan. Picture the scene: a person is stroking through the water, and with every movement of his limbs drops fly off in all directions. Wetting anything was never his purpose; the spray is nothing more than an unavoidable side effect of the act of swimming. Since no consent stands behind such water, food that it happens to land on remains as insusceptible to tumah as it was before.
Two separate points are folded into this ruling. The first concerns the water clinging to the swimmer himself. As long as he is still in the water, the water on his body is regarded as joined to its source, part of the very pool he is standing in, and water that is still attached to its source is not bechi yutan. Once he climbs out, however, the water remaining on him has been detached from its source, and that water is bechi yutan and can be machshir to receive tumah.
The second point concerns the spray itself. When he swims and the splashing water flies out beyond the pool, that water is not bechi yutan, because it did not leave the pool beratzon, with his will. He wanted to swim; he did not want water thrown anywhere.
When He Aimed the Splash
Now the Mishnah marks the boundary: "Ve'im niskaven lehazos al chaveiro, harei zeh bechi yutan" - should his purpose have been to sprinkle water upon another person, that water does have the status of bechi yutan. In this case the whole point of what he did was to place water onto something. Should any of that water leave the pool and reach food, it functions as bechi yutan, since the act was performed bekavanah: he purposely directed water at his fellow, and water sent on its way by such a purpose ranks as water that arrived through a person's will.
Blowing Bubbles in the Water
The Mishnah closes with a further case: "Ha'oseh tzipor bamayim, hanitazin vehashavim einan bechi yutan" - one who makes a "bird" in the water, that is, one who blows bubbles into the water. Here there are two kinds of water to consider: the water that sprays out as a result of the bubbling, and the water of the bubbles themselves that leaves the pool. Neither one is bechi yutan. His entire intention was to make bubbles. He was not trying to wet anything, and so even if this water reaches food it is not bechi yutan and cannot serve as hechsher to receive tumah.
The thread running through the whole mishnah is that water becomes significant only when a person wants it somewhere. Water still bound to its source, and water flung about by an activity aimed at something else entirely, remains halachically inert. The moment the person aims the water at a target, it becomes bechi yutan.