We have found out that "questions" are mostly useless for foreigners. You have to know the answer to ask the right question.
A flood occurred yesterday. The flood (previously mentioned) was very dangerous (new information).
We left Brasilia yesterday. Why? We were in danger because of a flood (you didn't know about the flood).
They have finally discovered the cause of the flood in Brasilia (we already know about the flood). It was heavy rains (new information).