Pesachim 33 - Squeezing pure juice out of impure grapes
Rabbi Yochanan made a puzzling statement: "If grapes became impure, one can squeeze juice out of them in the amount of less than an egg’s volume, and that juice will be so pure that one can use it for libations in the Temple." How could that be?
The answer is that Rabbi Yochanan considers grape juice as a separate liquid that happens to be contained in a grape skin. As such, the skin protects the juice rather than contaminate it. When he squeezes the juice out, given that the amount of grapes is less than an egg’s volume, it cannot transmit impurity further.
Still, Rav Chisda was amazed and asked, "The impurity of the grapes, where did it go?" That is because according to him the grape juice permeates the grape and becomes one with it, so an impure grape skin makes the juice impure right away, and this impurity cannot disappear.
Art: Hanging Grapes by George Henry Hall