Three categories are prohibited to a nazir: coming into contact with ritual impurity, haircutting, and eating grape products. All grape products [e.g., grapes, seeds, skins] combine to comprise the minimum quantity [prohibited to the nazir]. He is not liable until he eats an olive-bulk of grape product. They used to teach, “until he drinks a revi’it of wine.” Rabbi Akiva says, “even if he soaked his bread in wine and it absorbed enough to add up to an olive-bulk, he is liable."
Chapter six of Numbers introduces a type of vow in which a person voluntarily takes stringent ritual restrictions upon him- or herself. What motivates such an oath? It could be a feeling of guilt, or a desire to attain greater sanctity. The Rabbis discouraged unnecessary oaths and saw the nazir as an extravagant and somewhat unstable person. Perhaps his oath to avoid all wine products stemmed from insecurity about his self-control. The most famous nazir, Samson, was indeed an unstable person. Our mishnah documents a progressively stricter interpretation of the law to the point that even grape seeds, as well as wine, were forbidden.