Maaser Sheni, Chapter Two, Mishnah Ten
In this mishnah a man has sons, some of whom are pure and some of whom are not. He is in Jerusalem and he wants to buy the pure sons wine with maaser sheni money, and give regular non-sacred wine to his impure sons who are not allowed to drink maaser sheni. The mishnah finds a way for him to let them both drink from the same jug.
If one had some of his sons pure and some impure, he may put down a sela and say: What the pure drink, this sela will be redeemed for it. In this way the pure and the impure may drink from one jar.
What he needs to do is take the maaser sheni sela and say that it is being redeemed only for what the pure sons drink. In this way, only the wine that the pure sons take from the jug will be maaser sheni. What the impure sons take remains hullin, non-sacred produce.
Note that the impure sons do not need to touch the wine in the jug. They can pour wine from it and drink from it.