Maaser Sheni, Chapter Two, Mishnah 10. The Mishnah deals with a man some of whose sons are tamei and some of whom are tahor, and he wishes to give them a barrel of wine that all of them will be able to drink from.
The difficulty:
The father wants the wine that his pure sons drink to be purchased with maaser sheni money, since they are in Jerusalem and can eat it in purity. His impure sons, however, cannot eat maaser sheni. It turns out that if they all drink from the same barrel, part of the wine would have to be redeemed with maaser sheni money and part of it could not be redeemed, since the drink that the impure sons will drink cannot take on the status of maaser sheni because of their tumah.
The Mishnah's solution:
He sets aside a sela of maaser sheni and declares: "Kol mah shehateharim omdim lishtos" - whatever the pure sons are about to drink, let this sela be redeemed onto that quantity. In this way the pure sons are drinking maaser sheni.
The explanation of the Yerushalmi:
The Yerushalmi points out that this is not so simple: how can maaser sheni and chullin be mixed together in the very same barrel? Therefore the intent of his declaration is that whatever the pure sons actually drink - at that very moment, at the time of their drinking - the sela should be redeemed onto that wine. Only that the redemption must take effect from now, from the moment he makes the stipulation, and retroactively apply to the moment of drinking. The result is that they are drinking maaser sheni, yet the maaser sheni they drink was never mixed with the chullin that is fit for an impure person to consume. In this way both the pure sons and the sons who are not pure can drink from the same barrel.
The question of tumah through touching the barrel:
The Yerushalmi further notes that if the sons touch the barrel itself, we must be dealing with sons who are tamei mes, who do not convey tumah through hesset - that is, they do not render something impure by moving it. Likewise, we must say that the barrel is an earthenware vessel, which does not become impure from its outer surface, and therefore touching it from the outside does not render it impure. But if we were dealing with a zav, who conveys tumah through hesset by moving the barrel, it would be a problem.
Alternatively, one can explain that the case is one in which a third party pours the wine for them into their cups.