The Babylonian Talmud, tractate Shabbat, folio 21b, asks directly, "What is Hanukkah?" and answers with a story: when the Hasmoneans (the Maccabees) overcame the occupying forces and entered the Temple to rededicate it, they searched and found only a single flask of oil still bearing the seal of the High Priest, certifying it ritually pure, and it held enough to keep the Temple's lamp burning for a single day. They lit it anyway, and it lasted eight days, the time needed to press and prepare new pure oil. The rabbis, the passage says, established those eight days as a festival of praise and thanksgiving the following year.
This is the account behind the central home ritual of Hanukkah, the lighting of an additional candle on a menorah, or hanukkiah, on each of the festival's eight nights. It is also, notably, not the story told in the earliest surviving sources for the same events. The Talmudic passage itself is brief, only a few lines, compared to the extended halakhic discussion of candle-lighting that surrounds it in the same tractate, which some scholars read as a sign the story was included chiefly to justify a ritual practice already in place rather than as inherited historical memory. Whatever its origin, the story has shaped Hanukkah's central home ritual for over a thousand years and is the version most widely known today.