I don't think that this is a stupid question. The fact is, our so called Hindu books have represented Ramayana in several contexts that they have confused civilians and everyone is curious to know about the reality. I hope, I could throw some lights on it.
According to the legend (from the source of Balmiki written Ramayana), the kingdom of Mithila once fell under the curse of evils which subsequently caused the entire land being deprived from rain for years. This has adversely affected the Mithila as the absence of rain dried up the lands and rivers. This has affected crops terribly thereby causing a state of utter starvation in the kingdom. Later, The pious Janak thought about ploughing the land himself. Upon ploughing, the plough stuck to something, hidden in the soil. Upon excavation, the king found an object (which was a gold-decked small box). he opened it and found a beautiful baby child inside the box. She was a miracle lady as her appearance brought forth eternal prosperity to the Mithila. Since Janak found the lady inside the soil, he christened her SITA and adopted her as a daughter. Thus, the story of Sita being the daughter of Janak began, and since then everybody believes Sita is the daughter of Janak.
However, the question remained unanswered as to who buried the box? We find the answer of this mystery in a source (sorry, can't recollect now), that the king of devils, Ravana (also the king of Lanka), got several saints butchered mercilessly and stored their blood into a pitcher. When the pitcher brimmed with blood, he got it stored in a small box and ordered his troops to safely bury it somewhere in a deserted place. Coincidentally, the troops found a land (geographically located in the kingdom of Mithila), and buried the box there. The buried box cast an evil spell to Mithila. Thus the cursed land never met rain until the king, Janak, himself exhumed the box. Since the blood was from pious saints, it took form of a lady which later was called as Sita. Remember, those killed saints had cursed Ravana that a lady would be the cause of him and his entire kingdom's annihilation. Everybody knows the aftermath story of Ravan's devastation, started from the moment he kidnapped Sita.
Thank you very much for reading my post.