Explanation: פרק יב הלכה יא - The juice inside the grape/olive doesn't become טמא on account of the grape/olive themselves becoming טמא. When the volume of the entire grape/olive is an exact ביצה, the juice from it will be less than a ביצה. The חכמים enacted that for the juice to be טהור, the entire grape/olive must be less than a ביצה.