Hanukah celebrates the miracle of a flask of pure olive oil that burned for eight days. Since, the jar contained enough oil to burn for one day, the actual miracle took place for seven days. If so, why do we celebrate the miracle of Hanukah for eight days? The answer is that nature is not an independent force, detached from HaShem. Indeed, nature is bound by predictable laws, which are under HaShem's absolute control. The Almighty, alone, causes the sun to rise in the morning and the appearance of the moon and stars at night. At Hanukah time, we recognize the fact that oil fueling a flame is a miracle, in and of itself. Therefore, we light one additional day to thank HaShem for the miracle of "nature," that oil ignites a flame.
HaShem is the sole cause of every "natural occurrence." Everything in the world -- the birth of a baby, the daily sunrise, the changing of seasons, the combustibility of oil, etc. -- are all hidden miracles that HaShem, alone, constantly performs.
When you light your Chanukah candles recognize that HaShem makes oil ignite the flame. Look beyond the veil of "nature" and acknowledge that it is HaShem Who makes the stars shine, the trees blossom, and the rivers flow to the sea. Thank HaShem for the countless and continuous hidden miracles that surrounds us every second of our lives!
(Based on the writings of the Saba Kadisha of Kelm)