Yoma, Chapter Eight, Mishnah Two
This mishnah teaches the minimum amount of food or liquid that one needs to eat or drink in order to be liable on Yom Kippur. The mishnah is not saying that one is allowed to eat or drink a lesser amount, just that one who does eat a lesser amount is not liable for having transgressed a biblical commandment.
1) If one eats [an amount] the size of a large date, like it and its seed, or if he drank [an amount equivalent to a] mouthful, he is liable.
2) All foods add up to make an amount equivalent to a date, and all the liquids add up to make a mouthful. Food and drink do not add up.
Section one: When the Torah prohibits eating and drinking on Yom Kippur it uses the language afflict and not the word eat or drink. According to the rabbis this means that one has violated the biblical commandment only if one eats or drinks an amount of food that satisfies, at least in a minimal way, an average person. The rabbis hold that this amount of food is the size of a date with its seed, or in the case of liquids, a mouthful. We should note that normally one is liable for eating an olives amount of food (a smaller amount) or a reviit of liquid (more than a mouthful). For Yom Kippur the issue is not what is food but rather what satisfies.
Section two: If one eats two different kinds of foods or two different kinds of liquids they do add up to a date/mouthful. For instance if I eat half of a dates worth of a cracker and half of a dates worth of cream cheese, I am liable. However, food and drink do not add up, so that if I eat half a dates worth of a cracker and drink half of a mouthful of juice I am not liable