Machshirin, Chapter 5, Mishnah 1. Throughout this masechta the decisive question about any liquid is whether it came onto a person or an object with his consent. Water that a person is pleased to have is beratzon, and such water can serve as hechsher, preparing food to receive tumah; water he never wanted has no such power. Our mishnah opens the fifth perek by applying this to a man walking out of a river, and it produces a striking result: one immersion can cancel the halachic effect of another.
The opening words are: "Mi shetaval banahar, vehayah lefanav nahar acher ve'avar bo, tiharu shniyim es harishonim." Someone dipped himself in a river, found another river blocking his way, and passed through it; the second body of water has purified what the first left on him. Picture the scene: a man goes into a river to bathe and is glad to do so. Whatever moisture remains on his skin when he steps out is moisture he wanted, so it has the status of beratzon. Were drops of it to fall on produce, they would supply hechsher and make that food susceptible to tumah.
Then a second river lies across his path and he wades through it. He has no interest whatsoever in getting wet a second time; that crossing is simply something he has to do. The water of the second river therefore never becomes beratzon, and on its own it can give no hechsher at all. But the mishnah teaches more than that. As he passes through, the second river washes off the water of the first river and replaces it with water that carries no ratzon. The man emerges with nothing on him that can give hechsher, and the entire matter is now clear.
The mishnah continues with a parallel case: "Dachahu chaveiro lishkoro, vechen livhemto, tiharu shniyim es harishonim." After bathing in the first river, this man comes to a second body of water and his fellow, in a state of drunkenness, shoves him in so that he falls into the water without meaning to. The same applies if he had washed his animal and the drunken fellow then pushed the animal into the water. In both instances the later waters purify the earlier ones. Nothing about the second wetting reflects the owner's wishes, since it happened only because of the fellow's drunkenness, so the second immersion is not beratzon and it strips away the effect of the first.
The mishnah closes with the contrasting case: "Im kimesachakin bo, harei zeh bechi yutan." If they were playing with him, this is bechi yutan, it is considered water that came with consent. When two people are horsing around and shoving one another into the water, being pushed in is part of the game and is exactly what the participants have in mind. Since the plunge is wanted, the water of the second river arrives beratzon, and it too is fully able to give hechsher and enable food to receive tumah.
So the perek opens with a neat pair of opposites: the same push into the same river renders everything inert when it was forced upon a man, and renders everything capable of hechsher when he was a willing player in the game. That completes Mishnah Aleph.